ATI 4850 or ATI 5750?

Hi guys. I have to chose between these 2 cards. 4850 is a bit more powerfull but 5750 has DX11, that makes it futureproof. Also, I've been looking to both cards models in XFX and Sapphire and realized 4850 in sapphire has no heatsinks over the memory chips and 5750 in xfx has the same lack of heatsinks in the memory. Is that important? Because I'd rather buy an XFX 4850 with a large heatsink and pay a bit more than get a card is gonna be broken in a couple of weeks. Also, I'm buying one of those for OC'ing. Here's the rest of my system:

Mobo ASRock G41m-S Socket LGA775
CPU Intel Core 2 Duo E7500 @ 2.93 GHz 3MB L2 Cache
RAM Corsair XMS2 4GB 800Mhz
HDD Western Digital Caviar Blue 500GB SATAII
PSU Antec TruePower Trio 550w SLI Ready
Speakers Altec Lansing Subwoofer
Keyboard Logitech Classic 200
Mouse KLIP Xtreme KMO-102
Display AOC 17'' 1280 x 720