I just bought a FX5900 standard (400/850,2.2ns), which should mean lots of graphical bandwith increase, while, my MB/CPU/MEM are too old, say, ECS K7S5A (oc143Hz)/XP1800+(oc2400+)/512M HYDDR266. Do you think that the above system no longer matches the FX5900, because my aquamark3 benchmark is only 33000 (4X FSAA &4X AF&1024X768). I plan to upgrade my system. Does aN upgrade including XP2600 (FSB400Hz), Nforce2-400ultra MB and 512MB DDR400 fit the FX5900?
Actually, the K7S5A holds up decently if you're overclocking the FSB, which you are. Upgrading your system to an XP 2600+ would be a waste if you're already running an 1800+ o/c to 2400+ specs.
The CPU upgrade you've proposed won't do diddley in games because the AthlonXP rating numbers aren't really accurate as a performance measure after 2400+. The memory might help, especially if you run in dual channel mode, but it still won't justify the money you've spent on a new setup. I don't even think an an Athlon XP 3000+ would show you a big difference in actual gameplay.
If you want to see a real-world notable increase in gaming speed, you're going to have to go with at least a P4 >3ghz or an Athlon64.
The trusty 'ol AthlonXP won't cut it anymore, god bless it for lasting this long though.
By the way, your aquamark score is just fine with 4x FSAA and 4x Ansio. I'm not sure what you expect out of this card.