My rig,
AMD 6400+ Black Box Edition, zalman 9700 HS (can be upgraded to quad core)
ASUS Crosshair MB
4 Gig's ( 2 gig each) xms2 dhx matched pair 800 Corsair mem
( will take 8 gigs)
2 seagate sata's 320 & 500 Gig (will be changing to 2-500's or 2- 750's)
HP 20x dvd burner
nv BFG 8800gts512oc ddr3 vid
Sound Blaster X-Fi XtremeGamer Fatal1ty Professional Series sound card
antec 650 watt ps
antec 900 case, 4 fans (can add 2 more)
vista ultimate 64

I want to upgrade graphic card, which offers most bang for bucks?
Also should I take this rig to 8 gigs of ram?

If I remember correctly, that motherboard supports sli. If so, add another g92 8800gts, available for as low as $120 after mail in rebate. A single stock g92 8800gts performs about the same as 4850. 2 in sli will outperform every single gpu card in existance, including gtx280. Actually, it will also outperform every dual gpu card in existance so far, such as 9800gx2 and 3870x2, though not 4870x2 when it comes out next month.

I'd like to disagree on the performance listed above about it being so pwerful, but I'd say that another GTS is the way to go.

dagger has the best analysis

thanks a gts 512 is a lot less than a 280.

an 8800 gts is not comparable to a 4850. but yes another 8800 would be your best upgrade if your mobo supports sli.

are you running 64bit OS? if not your not even using all 4 gigs of ram.

Some benchmarks:
UT3
Far Cry
Crysis
Call of Duty 4
Supreme Commander
3dmark06

Another 8800 GTS 512 is the best option, the 8800GTS will outperform the 4850 (this fact may be covered by a shroud of hype and fanboyism, but it'll become more clear as time goes by) and in SLI they'll be faster than a GTX 280, as dagger aptly said.

I wouldn't say 8800gts outperform 4850 overall. That is only the case if you compare resolutions of 1280x1024. The one key advantage of 4850/4870 is it scales well as resolutions increase. Look at the 2048x1536 charts, the 4850 leads 8800gts in almost every game. Of course, in games like Crysis, the 4850 gets 13.6fps at that resolution, which is unplayable anyway. But it's still valid for performance comparasion purposes.