Dude, these video cards are really whats screwing you here, I'm running an Ati Radeon 4670 the 1GB IceQ edition, the card does have hiccups on Black Ops if you have it over med/high settings but other than that I'm standing around 60-70 FPS range almost all the time. Also if you would go for a Desktop, theres this build that I followed on the internet, and so far its great and includes this Video Card.
Case -
Newegg.com - APEX TX-381-C Black Steel Micro ATX Tower Computer Case
Mobo -
Newegg.com - BIOSTAR A780L3L AM3 AMD 760G Micro ATX AMD Motherboard (Crossfire compatible :O, If you wanted to, you could Crossfire two of these same cards I believe)
DVD Drive / Optical Drive -
Newegg.com - ASUS 24X DVD Burner - Bulk 24X DVD+R 8X DVD+RW 12X DVD+R DL 24X DVD-R 6X DVD-RW 16X DVD-ROM 48X CD-R 32X CD-RW 48X CD-ROM Black SATA Model DRW-24B1ST/BLK/B/AS - CD / DVD Burners
Power Supply -
Newegg.com - APEX AL-D500EXP 500W ATX12V Power Supply
Processor -
Newegg.com - AMD Athlon II X3 445 Rana 3.1GHz 3 x 512KB L2 Cache Socket AM3 95W Triple-Core Desktop Processor ADX445WFGMBOX (3.1 GHz Triple core :O, You can unlock for more I believe )
RAM -
Newegg.com - Crucial 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10600) Dual Channel Kit Desktop Memory Model CT2KIT25664BA1339 (You might want to go for the 8 GIG stick if you are a REALLY heavy multitask-er, Or just keep these 2x2gb DDR3's, because I don't think you would need any more than that if you aren't some kind of multitask-er)
Storage -
Newegg.com - Western Digital Caviar Blue WD5000AAKS 500GB 7200 RPM 16MB Cache SATA 3.0Gb/s 3.5" Internal Hard Drive -Bare Drive
Video Card -
Newegg.com - HIS H467QR1GH Radeon HD 4670 1GB 128-bit DDR3 PCI Express 2.0 x16 HDCP Ready CrossFireX Support Video Card (NEVERMIND, Newegg is being a bitch, this card is out because it is such a great deal, but! This is also where you would want to invest more, but make sure the PIN setups match with the MOBO, you don't want to be buying a giant video card you can't use.)
The COST: Somewhere around 500, and that depends on if you need an Operating System, if not this is around a 400 dollar gaming computer.

Hope I helped, All credits @ Multiple websites across the internet and multiple reviews of video cards on youtube.