Blade:
Will give you two arguements, you can make a choice:
Scenario 1:
The 4870 is a better bet right now for you. It will thrash the games on your 20" (except Crysis). Getting a 5xxx series card as of now is pretty useless. They are way too over priced due to low supply and high demand. There are not even 5 Dx11 games out as of now. It will take a while for these games to come out. And even then, how can you be so sure, the next or subsequent Dx11 game which comes out -- will be of your liking --- unless you play 'every' game which is released.
The 4870 packs more punch then the 5770. 1.5-2 years down the line, when ever thing settle down (Fermi, Dx11, 5xxx, Dx11 games) you can swap cards.
Scenario 2:
The HD5770 is a good in one scenario. And probably the best here. Xfire. If you Xfire this card, it kicks the living daylight out of almost every card on this planet. If in the future you really plan to go Xfire, then only buy this card now. You are making a good investment this ways. Once you have more money, you can install another card. And believe me, you will have a whoooping fast system -- with tons of thump..!
You want a system that you will not upgrade in 4 years. That is currently difficult to manage. You PSU is low. Just the bare minimal. Down the line (4 years), you will probably put in more stuff...it will strain. Motherboard, if plan to go dual GPU -- you have a single PCIEx16 slot.
