Having been researching this for hours, I have thoroughly confused myself and have decided the only way my question is going to be answered is if I join a forum and specifically ask it. So here I am. And here is my dilemma:

I have a Dell 8400 with a Pentium 4 (Prescott) 3GHz (800MHz FSB) purchased in August (or July maybe) of 2004.
I am upgrading from 1 gig (2x512 DDR2-400 CL3) to 2 gig (2x1)* and am trying to decide between DDR2 400 with CL3 (http://www.newegg.com/Product/Prod [...] 6820134181) or DDR2 533 with CL4 (http://www.newegg.com/Product/Prod [...] 6820146569). Cost is not an issue since there's only a $2 difference. Most of what I have read focuses heavily on overclocking, which is irrelevant in my case (because it's a Dell) and something I wouldn't want to mess with anyway. I also read a lot dealing with AMD cpus and Core2Duos and irrelevant RAM (like older DDR1).
And something about 1:1 bus:ram ratio being good, which means the 400 would be better. I have no idea why. Is this significant?

So I guess my main question is which would be better for gaming and/or multitasking (multiple large Photoshop and/or Illustrator documents, Video editing, etc.) Which has a bigger performance impact: the CAS latency or the frequency? I know lower CL is faster, but how does the extra 133MHz and lower CL of the PC4200 affect performance? Is the extra bandwidth worth the higher latency? My current memory has a CL of 3 (according to cpu-z), btw.

*A third option is to go the cheaper route and add 2 more 512 sticks (http://www.newegg.com/Product/Prod [...] 6820144166), but I'm a bit nervous about mixing brands and I think 2x1 is supposed to be better than 4x512??? Something about the memory controller and voltage, I think. Any thoughts on this?

Thanks.

Edit: Crucial site shows DDR2 667 and 800 as compatible, but those weren't options when I bought mine, so I assume my motherboard doesn't support those speeds.

Higher frequency is better then lower latency by a longshot. However you will only notice performance gains through frequency until your ram is half your CPU FSB. So in your case DDR2 400. DDR2 400 in dual channel gives 800mhz bandwidth which gives you the 1:1 ratio with your CPU FSB of 800mhz.

CAS Latency makes a very small performance gains and in most real world scenarios is not noticable. Many overclockers will infact increase latency to get a higher frequency from memory.

DDR2 667 and 800 will work in your board but will only operate at ddr2 400 speeds. If you plan to build a new system soon i would get a good set of ddr2 800 as you can use it in a core 2 setup later on. (that is if you build before intels next chip nehalem.)

I had an E520 dimension, best thing i did was to buy a new case,mobo and psu and swapped the guts over from the dell, You can pick up a E4*** cpu cheap which will destroy the Pentium D cpu u have at the moment.

Dont waste your money on Dell propriety stuff.

Thanks, I guess I'll go with the 400. As far as upgrading, I had planned on getting an entirely new system and keep my current one intact for compatibility reasons, though you've managed to get me looking at mobos on newegg.

Well, I changed my mind. I think I'm going to begin my first full computer build by buying this ram: http://www.newegg.com/Product/Prod [...] 6820146764 Hopefully it will work in my Dell until I can accumulate other parts. I also hope my power supply (Enermax eg495 485W) will work with a C2D e8400.

Your PSU is fine, It will power an 8800GT as well.