Ok, To me an upgrade is only worth it if your current rig is limiting you from enjoying your games, and you have looked into what upgrades will clearly help better your gaming experience. Basically, Video Card, CPU, and enough memory are all key. Not sure what you had before, but that 6800GT probably a very good move for increasing the look and smoothness of your games. Your XP @ 2.1+GHz isn't all that bad really, how much ram do you have?
You've already heard that the A64>P4>AXP for gaming. This is true. But let me ask exactly why you want to upgrade now? What games are not as playable as you would like? How much Ram do you have, as 512MB is the minimum, and some games clearly benefit from having 1GB of ram? Or is it benchies that make you want a faster CPU? Is you overclocked system stable? Is it too loud for your liking? Basically what is wrong with your current rig?
The reason I ask, is while A64 is clearly the way to go if you are going to upgrade for gaming; do you need it? Alot of people upgrade because they see others with more 3dmarks or Aquamarks. Others think that they will see a world of a difference in every game. Which isn't going to happen in your system. In your case, which is it?
Hardocp had a good doom3 hardware guide, but their site isn't up right now (Sat morning), so I'll link to Anands Doom3 CPU review and Firingsquads HL2:VST/Source tests.
To me Xp3200+ is about = a P4 2.8C, which not all that bad for gaming these days. Stepping up to an A64 3200+, like has been recommended, gains you alot in benchies run at 800x600 where it's totally cpu limited. But step up to the resolutions you will probably game at, and the differece is more limited by the 6800GT. For instance, as a guestimate/example, You'll be spending alot to go from 55fps to 65 fps at 1280x1024 High quality in Doom 3, or from 50fps to 53 in HL2/CS:Source at 1600x1200 6X AA/8XAF. Not a big deal really, if we were talking 30fps to 50, that's different. But as resolution and Quality settings get cranked, the limit is more and more on the GPU.
I'm not knocking A64 as a far quicker gaming chip, but the differece you will see at gaming resolution over your AXP@2.15GHz, isn't that great. As long as you look into this and understand that for gaming (depending on the game) you might not see much(any) of a difference, but for game benching at certain resolutions or synthetic benchies you will be able to discover a big difference. To me not worth the upgrade just for that.
I'd hang onto that current system until you can afford to make a huge increase in cpu performance. Or like me, a deal comes along you just can't pass up. I understand you have a nice AGP card so you want to pair it with a high end system that can last as long as possible, so waiting for NF4 mobo with PCI-e doesn't make sense. But I can't see that your gaming expereince is so crippled to upgrade every year. I'd either go with a very fast cpu & AGP mobo now if you must, or wait a while for the price to go down on one. Forget P4's or A64 3000/3200+ unless a true baragin falls in your lap. Just not a big enough differnce right now to justify the money/swap. If you just want an upgrade for the thrill of it, A64 3200+ on a nice mobo will do better than anthing in it's price range.
ABIT IS7, P4 2.6C, 1GB Corsair XMS 4000 Pro Series, Radeon 9800 Pro, Santa Cruz, TruePower 430watt
yeah I get completely what youre saying, its just I have no life and my main hobby is my computer... Its just cool to have something fast, or a 64bit cpu, etc. I only play Roe:Total War nowadays and admittedly it runs just fine... I dunno, computer is my hobby its really all I spend money on. It just seems so right to upgrade.