If it is for your computer i wouldn't worry about top end graphics cards...
IMO the difference will be minimal, so i guess get the cheapest one if you must...
I may be off base but if you have the sort of cash those graphics cards cost, to spare, then sort out your computer with a decent upgrade. I won't mock anyone who has an old computer and can't afford a new one, but you seem to have the cash and ignore every part of the computer except for the graphics card.
Top of the range graphics cards and P4's even @ 4Ghz with 1gb RAM do not belong in the same sentence.