from the link u gave at dell,select another video card,ur current selection is Intel....which suxxxx for a gpu,choose one of the atis,but actually for best computer configuration u need to choose each part by yourself

I can give you a start
best price/performance charts

select CPU,GPU charts
currently best CPU is AMD Athlon X2 4800+
best GPU is GF 9600GSO as you can see..
If you choose these just search for a mobo with AM2 socket(the athlon's socket)and to have PCI express not agp

also,you can check tomshardware
click charts and choose which chart.
for Ram I recommend you to go with DDR2-800mhz,its best for now.DDR3 wont increase ur speed much and its way too overpriced.

for HDD choose 320gb,Seagate,SerialATA,7200rpm,16MB(seagate barracuda 7200.11)its best read/write speed in world and its very cheap

for PSU get a normal 400W or 460W

I hope I helped you