a properly functioning 9800pro will run HL2 on High. I tested a 6600GT in the past, and it ran HL2 on High/1024x res/2xAA.... A 9800 pro could probably do the same, except with either 1 lower Resoution setting, or without AA.
a 9600, or 6600. would probably run HL2 on medium.
Check out EVGA 7600GS or 7600GT though, on Newegg.com ..... they have the E-VGA 76GS for $110 after reabte, and the 76GT for $170 after rebate.
the 7600GS is around or better then the stock 6800 performance, and the 7600GT is better then the 6800ultra performance.