512Mb of RAM is plenty enough.
I would have to agree with pretty much everyone.
Your video card is quite good enough though, so I would lean more towards getting speakers.
I highly recommend Logitech's Z-680 or Kipsch's Promedia Ultra 5.1 speaker sets.
And since getting either of those would leave you with plenty of money for RAM, I guess you could throw that in, too.
NZXT Guardian Case with Thermaltake PurePower 420w
FIC AU13 motherboard
AMD Athlon XP 2600+ 333FSB (Barton) with Thermaltake Silent Boost HSF
WDC 80Gb 8Mb cache 7200rpm
512Mb Kingston Dual-channel PC3200
MCP-T SoundStorm audio (integrated)
ATi Radeon 9800 Pro (it owned, until x800 and 6800)
Logitech Z-680 5.1 speaker system (integrated Dolby Digital 5.1, DTS, Pro Logic II decoding)
Logitech Cordless Elite Duo keyboard & mouse
Saitek Cyborg Evo joystick
Windows XP Pro SP1 (dont. install. SP2.)