Radeon HD 5770 1 GB is the current Best PCIe Card For ~$130 according to TH* in the same "class" as GTX 460 768 MB which means it should deliver great 1920x1200 performance in most games.
Novatech does them for £97.98, Aria £99.00, eBuyer £99.99, Scan £101.48 and Play £116.09. If you can get them as second hand on eBay, so much the better.
As always, check that your PSU is up to snuff.
Graphics Processor : ATI Radeon HD 5770
Memory : 1 GB GDDR5
Bus : PCI-Express 2.0 x16
Outputs : Dual DL-DVI-I