You cant do it man.
1: Your PSU is weak and not upgradable.
2. PCI video cards are very poor.
Save your money and build a system. One with AGP or PCI-E, then you could upgrade in the future without any trouble. And DONT buy a Dell.
Intel Core 2 Duo E6600 @ 3400Mhz
BFG 8800GTX OC
Samsung 400 GB Sata II
Corsair XMS2 2GB DDR2 800 5-5-5-12
Zalman CNPS-9700 LED
Antec Nine Hundred
Hp W2207 22" Widescreen Monitor
Windows Vista Home Premium
Silverstone ST60F 600W PSU
Razer Copperhead Tempest Blue Gaming Mouse