I have a strange issue that I'm having a difficult time resolving. I built my system years ago, and upgraded it recently. And now I'm having issues with 3DMark03 testing and World of Warcraft. My graphics cards settings and in-game video settings match the old specs.
 
The "before" specs:
- AMD AthlonXP 2800+
- GeForce 6800 GT
- 1 GB RAM
 
The "after" specs:
- AMD AthlonXP 3000+
- GeForce 6800 GT (no change)
- 1.75 GB RAM (replaced a 256 MB stick with a 1 GB stick)
 
Motherboard (no change): Asus A7N8X-E Deluxe
 
The upgraded system has a new hard drive with Windows XP SP2 installed fresh and updated. The old hard drive is slaved and set to drive F: so I can retrieve old data as necessary. In addition to SP2, I installed the latest nVidia ForceWare drivers, and the latest mobo drivers for my system.
 
On my old system, my average 3DMark03 (which I've kept using for consistency) test score was around 10,500. The new specs produced a comparitively less impressive score of about 7300. I later learned that I was underclocking my CPU and my AGP aperture size was too low, and when I fixed that up my score rose back to about 10,700. Things were good for a while and WoW ran fairly smoothly.
 
Last night, before bed, I found WoW to be choppy. To describe what I mean: the game ran fine for 1-4 second bursts, and in-between those bursts, for about a second or so, the frame rate dropped in half. It happens in heavily populated areas or in barren fields, whether or not my character is moving. I log out, run a 3DMark03 test, and get an abyssmal 6603 score. I noticed the test was exhibiting the same symptoms as WoW: momentary choppiness. I did a quick CPU-Z test to ensure my CPU multiplier hadn't dropped, and it is still at 12.5x where I left it. Unfortunately, I hadn't the time to check the BIOS settings, as I needed sleep. I'm typing this from work this morning.
 
I did check my CPU and GPU temps, though. My CPU runs about 48C under load, and my GPU about 65C, again under load, both well within acceptable levels for the respective temps. (Yes, I know that the CPU is a bit hotter than most techs like to see, but an AthlonXP can survive that temp under load. GeForce 6800 GTs run pretty hot, too). These are cooler than my old setup, which worked fine.
 
I will recheck the AGP aperture and other BIOS settings this evening, but I suspect they are going to be the same. If so, I'm not sure what else to check. Please advise, and thanks in advance!

I reckon it's your RAM... If I were you I'd get rid of the smaller modules and have two identical 1GB's in there.  
 
Chances are the latencies of the RAM modules are totally different, as will be the SPD's, possibly clocking down your fastest Ram to be inline with your slowest... This could be the cause.  
 
Also, Power supply... big one here. Always have a quality one fitted. The extra HDD may be adding strain to the PSU... even though its not a great deal of extra power being used it still might be causing the adverse effects.  
 
Try running the computer with just 1GB Ram fitted. Try out your 3DMark03 and get back to us with your score.

Yeah, the ram is the first thing I would check.  Like dev1se said, having mismatched sets of ram can do funny things.  

Much thanks for both of the replies. If I cannot find anything wrong with my BIOS settings tonight, I'll try having just the 1GB of RAM.
 
It is still strange to me that after I first stopped underclocking the CPU that everything worked just fine. If the mismatched RAM were the issue, wouldn't it have reared its head back then?

Yeah and if it performs better on the benchmark, buy another identical stick of 1GB Ram... 2GB is the sweetspot for Windows XP so you'll notice the most performance gains at that point.

What about your PSU?  What are the specs on it?  Like it was said above, since you added another HD I would make sure you have enough power too.  

I don't remember the brand off the top of my head, but it's a 430W PSU. I purchased it about a year and a half ago.

So on my way home I purchased another 1 GB stick of RAM. Same make, model, speed, etc. CPU-Z shows them as virtually identical. I also made sure my aperture and other BIOS settings were proper. Good to go.
 
No luck. My test returned 6607. A little stumped.

do you think it could be you're video card that is crapping out?(might want to test with another card if you can) I doubt it is though i would like to believe it has something to do with your power. Also did you check your voltage settings in the bios?

If your PSU is a cheap brand 430 watt, it is possible that it is either underpowering everything or is just dying.  I think the 6800 needed a 450 watt psu.  Adding the extra HD might have pushed it over the edge.

Okay, I removed the old HD and retested. 6600 even this time. So no improvement.
 
Again, this only began recently; before then, after I fixed the underclocking issue, everything was fine. I'm beginning to think the video is crapping out, but it seems rather odd still. Anymore ideas/suggestions would be welcome.

Not too long ago I was pushing my system with a no name 300W PSU.  There were 2HDs, 1 DVD burner, an AGP 6800GT, P4 Prescott, Xfi extreme sound card, wifi card, and 4GB of RAM.  On top of that, I even spliced into the frickin wiring to put in an extra 4 pin connector when I put my 2nd HD in because there wasn't enough to go around.  
 
Edit:  And yes, the system never crashed or had bizarre behavior.
 
There is no way in heck, that your system is under powered with a 430W PSU if mine was running like that unless it is really really horrible in quality but I would put money down that my PSU was crap compared to yours.
 
Maybe something is dying.  Does everything with your video card seem normal in GPU-Z?  Is it running at a clock of 350MHz with 1000MHz memory?  You are at AGP aperture size 256MB?  Your RAM is running at the correct speeds?  Your CPU seems normal?  

My AGP 6800U had a claimed requirement of a 480W PSU and needed two molex connectors.  Quite a power hungry card back in it's day.  I ran an Antec Truepower 430W at first but then with two HDD, two opticals, a bunch of fans, and overclocking I needed to upgrade and put in a Fortron Blue Storm 500W.  
 
Some 430W power supplies provide didly on the 12v rail.  Thermaltake for one had one with a single 16 amp 12v rail.  Anyway, it is quite possible an aging 430W could no longer power that upgraded rig.

Yeah, do you have access to a different PSU?

No access to a different PSU at the moment. If necessary I can find and purchase a new one.
 
EDIT - I opened up my system, gave it a once over with some compressed air (very little dust had gathered since the upgrade, but you never know), unplugged the card, reseated it, reseated the RAM, rebooted, went over my BIOS again, tweaked some of the settings in the nVidia control panel, tried again, and... 10,540.
 
So either one of the things I tried was the problem, or this is erratic and may turn up again. I'll keep an eye on it.
 
Thanks so much for all of your replies! If I ever take over the world, y'all can have your choice of island.

Some more testing, and while the benchmarks are now consistently at the 10,500 level, WoW is still choppy. I haven't installed any other games yet. I'll try that first and see what happens. Thanks again for the help everyone!

Make sure you don't have AA/AF fully maxed.  Use maybe 2-4x AA and 4-8x AF.  Also, in WoWs settings, make sure you keep the draw distance down a little bit (like 50-75%).  I have a friend who has a A64 4000+, 2 gig of ram and a 6800 Ultra that can't even run WoW at max.  Your GT is a little slower, as well as the CPU being a whole lot slower.

 
If you plan on taking over with a PSU, you may need to borrow someones