4850's crossfired would be quite powerful, equivalent to a modern day $300+ 5850 despite only have 512MB of ram. Since it is an older card, perhaps you can find one used?
Tom's Hardware
1680x1050 no AA Sum of Benchmarks VGA CHART:
http://www.tomshardware.com/charts/2...1050,2258.html
ATI Radeon HD 4850 CF
(2x512 MB)
682.60
ATI Radeon HD 5850
(1024 MB)
670.70
Tom's Hardware
1920x1200 4xAA Sum of Benchmarks VGA CHART:
http://www.tomshardware.com/charts/2...1200,2261.html
ATI Radeon HD 4850 CF
(2x512 MB)
405.80
Sapphire Toxic HD5850 2G GDDR5
(HD 5850 2048 MB)
404.80
I'd say it's value for money versus buying a new $300 card to just spend $150 to CF them. The only area of concern is that the 512MB of ram will limit your ability to use higher resolutions and higher levels of AA in addition to the maximum texture sizes of some games like Grand Theft Auto 4 and Starcraft II or if you like to use custom texture packs for some games like Oblivion/Fallout 3, etc.
Use the saved $150 to buy yourself an SSD and you are off to the races. You could also use the money saved to buy a better cooling solution and overclock that Phenom X4 9850 to around 3.0 GHz.