What to buy?

with a 15' monitor? The 5770 is WAY overkill for that. 4650 may even be overkill...

5770 would be a good choice if you are planning on upgrading your monitor to something in the 20-24" widescreen variety. Otherwise, it's way more than you'd need. You could probably do a 5570 or something like that and be ok.

A 5750 will be plenty for a long long time.

Cheers!

Please also post the rest of the specs for your PC, that has plenty to do with the selection as well, most notably the power supply specs and what CPU you are using..,.