ATI radeon HD 5670
This is a pretty big bottleneck. I don't know how much you know about numbering systems for GPUs but basically, a higher number doesn't necessarily mean better card. The 5 means that it's part of the 5000 family (Evergreen) of Radeon architectures. Inside of each family though, the remaining number (i.e. 670 in radeon 5670 or 870 in radeon 5870) will determine the performance ranking of the card. The 6 means it's a 5600 series, which is a particular form factor and architectural specification. The remaining 2 digits represent the model number, which will tweak smaller details like the clock speeds and the number of stream processors.
In short, a Radeon 4870 is a much better card than a Radeon 5670, because the 4870 is a high end GPU from an older generation and the 5670 is a low-mid GPU from the following generation. The only weaker Radeon cards in the 5000 family are the 5500s and 5400s which are low profile entry level cards designed basically to handle windows tasks and movie playback - basically graphics cards not meant for gaming. The 5600 is the bottom tier series strong enough to run current games on low settings and resolutions.
Not sure how much you're looking to invest at the moment in a new GPU, but I'd recommend never spending less than $150 on a new graphics card to play games with. Around ~$200 upwards of $250 at the high end is what I recommend to people wanting a strong GPU that will perform well and last for years but are trying to keep a realistic budget. The Radeon 6870
is my pick for that range, it's one of the strongest cards on the market that is still in that reasonable price range. A comparable nVidia card right now would be the GTX 560 Ti. Edited, Jan 31st 2011 3:01pm by bsphil