ATI Radeon HD 4850 is a good starting point.
I've got a ATI HD 4870 1GB and it runs SH5 easily.
tomshardware is a good starting point for reviewing/reading up on GPU's and benchmarking is also present with each card reviewed:
http://www.tomshardware.com/reviews/...card,2569.html
I won't go into any detail about what you should or shouldn't be looking for when buying a GPU, toms covers all that.
At the end of the day, you need to ask yourself 2 questions:
1) What's my budget
2) What will i use the GPU for? (ie intense gaming, graphic-driven workloads, etc etc).
Hope that helps
