-The x2-4400+ at 2.2ghz is not even close to a core2 at 2.33ghz. its more like the core2 based pentium dual cores at 2.0ghz or a pentium D at 3.8ghz. The cpu's are not the same per clock.
I have a X2-6000+ It runs at 3.0ghz. I remember the one up from your cpu since it was the competing cpu at the time I bought mine. I almost bought the intel e6600 that ran at 2.4ghz. The core2's had a slower clock speed back then, were a bit more expensive. Both cpu's overclocked poorly. The core2 was faster clock for clock, so a 2.33ghz core2 was about the same as a 2.8ghz X2 athlon64(5800+). I orginally bought the then new x1950pro with my system upgrade. A bit later i bought a used 4870-512 when it was only about 6 months old from its release. To date the only game that is cpu limited is dirt2 that gets about 30fps@1920x1440 2x msaa/med-high settings. However the dirt2 game favors a core2 so you would get double the fps I get. Games often favor either amd or intel cpu's. Games like fear2,left4dead2,ut3,batman,prostreet all run good on this computer.
With ati i would stick with the proven 800 shader cards like my 4870.
I seen the hd-4850 on sale for under $100. There is a new 1gb ddr5 4850 card out that almost matches my older 512mb 4870. It is around $100 too.
If you get a 4850 make sure you get the 1gb version, most are ddr3 that is still ok. The 4870 runs too hot so I would get the 5770 instead even if the 4870 is a bit cheaper. The 5750 also runs much cooler than the 4850 so if they are close in price the 5750 is better. The 4850/4870 cards overclock poorly where the 5750/5770 cards overclock more and run cooler.
The only card nvidia has is the $200 geforce460(overkill but would still work ok). The 9800gt/gt240/5670 are too slow.
I did see some 250's for sale for around $100 too, they are close to a 4850/5750, a geforce250 or 260 would be ok if you see one on sale.
I would say that the 800 shader 4850 or 720 shader 5750 would be a close match for your system, or a geforce 250 or 260.