I ran the benchmark and got a 4800 on low settings. I know that this will be fine but, I was hoping to at least get above 5k. I have been looking at the gtx 470 and the radeon 5870 both of which have varying reviews. My main concern is will I get a noticeable performance increase from a gtx280 to a gtx 470/radeon 5870/5850. From what I have been gathering the processor isn't a HUGE factor so I felt I could possibly OC my 920 to 3.6gig and save a upgrade there (think thats worth it). I have always been a nvidia gpu user so It has been a struggle trying to relate performance between that and a ATI.
If your goal is simply to get over 5k on the benchmark on low, it could be simpler than you think.
First, download CPU-Z and GPU-Z and check how much your CPU/GPU are being used while running the benchmark. I'm going to take a guess that your CPU will be running fairly high and your GPU would be running pretty low (running the benchmark on low is more CPU-dependent than GPU-dependent)...in which case would mean that the bottleneck is your CPU.
If this is the case, simply get an H50 liquid cooler for your CPU (have someone install it for you if you don't know how) and OC your CPU to 4Ghz.
This will put your benchmark score to over 5k. No need to buy a new CPU or GPU.
Check this out: http://img149.imageshack.us/img149/3729/cpuscaling.jpg