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Is there one that's 3GHz? It's hard to tell on different sites, but I'm not sure if Intel ones are compatible with my mobo.

The FASTEST AMD chips currently out are 2.4GHz. That said, there's more to AMD than clock speed. That's why their chips are given numbers (3500+, etc) that roughly equate to P4's clock speeds in MHz.
So if you're looking for an AMD chip that's about as fast as a P4 3Ghz, look at the 3000+ - 3200+ AMD chips.
Now if you want to reuse your existing mobo, you won't be able to go to the AMD64, you'll still need an Athlon XP. Go to the website for your mobo's manufacturer and look the model up. They'll list what CPUs are compatible.

Thanx, I just think for stuff like Doom 3, my system (especially RAM) is lacking.

for the near to mid term future there is no advantage to a 64 bit processor on gaming machines... if you can stretch what you have for a year or so, the newness (and therefore pricey-ness) of 64 bit systems will have worn off and they will be a bit cheaper - and there may be some software readily available to take true advantage of these systems....

now, if you have an application that is ported or you are going to port to 64 bit, then they offer significant perfomance increases over 32 bit processors....

my vote, if you are in any way limited by funds, is to wait until AMD has a 64 bit/PCI-Express story, then go for that and a new video card that is PCI-Express. if you have money to burn, just get the patest and greatest stuff right now (PCI-Express video card and EM64T mainboard and processor) and swap it again in 3-4 months (probably for an AthlonFX based board with PCI-Express)....and then upgrade your OS (reinstall) when 64 bit apps come out...

but really - are games even going to be 64 bit? they don't even multithread them right now...(granted, 64 bit would probably be an easier code than smp).

The draw of the AMD64 isn't 64bit. That's just something for a nebulous future. The draw is they play games faster.
Now for other types of apps P4's are better thanks to hyperthreading and such, but for regular games AMD64's are the way to go.
Check out these benchmarks in several games:
http://www.hardocp.com/article.html?art=Njc1LDU=

That's why my next CPU will be an AMD64. Oh, don't forget heat. The P4's run far hotter and take more power than a 64, so you need a beefier PSU and better case cooling.

Well, I have a fully rigged out server case, so cooling shouldn't be a big problem!

Yeah, I just keep mine in the fridge beside the beer.