SN45G Upgrade Question

Ok, need your help everyone. I have a SN45G 1st Version with an Athlon XP 1800+, 1GB of PC2400 RAM, and an ATI Radeon 9800 (not Pro, just regular). Anyways it is not NEARLY fast enough for the games I like to play (Oblivion, Battlefield 2142, Vanguard, Company of Heroes, etc. etc.) and I need to upgrade. I'm wanting to upgrade so my system will last another year or so. I'm planning on getting an Athlon XP 3200+, Two 1gb Sticks of DDR400 PC3200 RAM, as well as a new VGA card, which will probably require the PC50 300W PSU upgrade, as my current power supply is only 200W. So, after I have 300W of power and these new parts, what graphics card should I get? It has to be AGP, can only take up one slot (not much room inside these things), and must be a significant upgrade from my current 9800.... I have to run all the settings at their lowest to get a decent framerate in todays games.

Thanks!