Hmmm, im kinda in the same boat... Im trying to build my mavhine up to mid level performance... I got a really tight budget to work with... my specs so far is: intel 2 duo @1.86ghz, 4 gigs ram and rad hd 4650... ran 1550 on benchmark... i all ready order a new cpu (intel 2 duo E7400 @ 2.93 ghz) and i was planning on getting the rad 5670 for my gpu... i not looking to run the game max out on high setting, more like mid to high on low setting.. being that my monitor is only 720p.... so is this good enough to hit what im looking for?
I won't give you direct advice, but I will tell you what I plan on doing since I am in about the same baot as you.
I have an C2D E6600 OCd to 3.0GHz and an 8800GTS 320MB. I score about 2400 on low res.
My CPU is going to bottleneck any real GPU upgrade I might do. If I get a used GTX 260 for $130, or a 5770 for $170, or a GTX 460 1GB for $230, my score will max around 3500.
If I were ONLY going to upgrade my GPU, I would be wise to buy the cheapest option to get my PC to its max score of about 3500 and buy the used GTX 260 for $130.
However, I might want to buy a completely new PC in a few months if I am unhappy with the performance of this upgraded PC. If I do end up buying a new PC I will be upset that I essentially wasted $130 on a GPU I would not want to stick in a new PC.
So I plan on buying a GTX 460, and if I want to buy a new PC in a few months I will be happy to use that GPU in a brand new build as the only GPU or as part of a SLI pairing.