The fx-51 is 2.2ghz and uses duel channel ram(giving it twice the bandwidth);but also can use only ecc ram(which is more stable,but also a little slower than its clocked speed;because of the greater bandwidth it should perform much better under an actual 64 bit OS.
The Athlon64 uses single channel ram,supports only 1 gig of the fastest 400ddr ram-it's its inferior bandwidth and limited ability to support more ram which mainly make it inferior.It is mainly locked(limiting overclocking),unlike the fx-51 which is unlocked and can be overclocked.The mobos for the amd64 are also more numerous and cheper than those for the fx-51.
Really,there's no good cpu for a value built right now,by value I mean a system that will be great for a year,good for two more and ok with overclocking for two more.The amd64 does not have a lifespan of more than half that,the fx-51 will really be showing its age when real 64 bit systems using ddr2 and pci express come out and the intel cpu are not 64 bit capable;but the fx-51 is the best out there if you can afford such an expensive gaming-workstation and are willing to build again in a year.With both of the 64 bit cpu system you'll have to upgrade the GPU to a 64 bit system.I would say wait a year to a year and a half for a value build which will have a 64 bit OS, GPU/video card and ddr2 or at least wait 6 moths for the new amd socket 939 supporting non-ecc duelk channel ram or intels yamhil/prescott.
Thanks, great answer!
I'm buying one now because there's always another new technology around the corner. It's time to let Intel know what the market wants by voting with my wallet. Maybe I'll even buy two.
AMD64 dont bother really while the AMD FX51 is if you got the money. May just wanna wait for the next socket release, other wise get yourself a good Barton or t-bred chip and have some fun OCing the shizzel out of it.
Barton 2500+ @ 2200mhz (10x220 vcore @ 1.8)
Asus A7N8X Dlx 440 FSB
1gb Geil GD pc3500 Dual Channel (2-3-3-6)
Segata 80gb SATA 8.5ms seek
ATI Radeon 9800 Pro(420/720)