Just another stick of RAM.
The system you have is a good one already, and for the most part it should last you awhile as it is.
If you were to upgrade your motherboard, the only valid reason I could see, would be to get an 8x AGP slot for your 9700.
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.)