|"but why look for expensive ones and not cheap low power medium end ones like the hd 4670"|
Maybe to get more than barely acceptable performance out of a large monitor?
Honestly, I'd aim for at least an HD4850 or 9800GT to put in a gaming box. These cards are far from expensive and will provide a much better gaming experience.
Turbocharged 939 | Dual-core Opteron 185 @ 3.2GHz | 2x 8800GTS in SLI | 4GB PC3200 @ CL2 | A8N32-SLI Deluxe |1TB | PCI-E X-Fi Titanium Pro | Swan D1080MKII + Dayton 10" 125w sub | W7 Ultimate