the $900 set-up has these specs.
Genuine Windows® 7 Home Premium, 64bit, English
Intel® Core™ i7-920 processor(8MB L3 Cache, 2.66GHz)
3GB DDR3 Tri-Channel SDRAM at 1066MHz - 3 DIMMs
500GB 7200 RPM SATA Hard Drive
16X DVD+/-RW Drive
ATI Radeon HD 4350 512MB
Integrated 7.1 Channel Audio
Dell USB Consumer Multimedia Keyboard
Dell Studio Optical Mouse
Integrated 10/1000 Ethernet
what i see from the specs is that the gpu is the weak link.
i'd get the gtx260 gpu instead if you are gaming at resolutions higher than 1440x900.