You have quite a nippy system, the graphics card being the only 'let down' as it were.
However for the uses you have listed it is quite adequate really. However if you do wish to upgrade it, possibly to free up some of your system memory I would suggest an Nvidia Geforce 7300 series or 7600GS. Equally from ATI a Radeon X1300XT or X1650Pro. If you want to future proof your system to a limited extent a Nvidia 8500 would suffice as it is a brand new model supporting DX10.
All of the cards listed are budget or low end mainstream graphics cards. They should be suffice for your needs, and will even provide some power for a bit of gaming- they will also free up sytem memory. Speaking of which, considering the pcs useage I would not pay extra for more video ram.
If you link any graphics card you were considering we can give you further advice. Hope that helps.