That's a tough one. The 1Gb of the current 6950 is going to hold you back in some cases. I'd say it would be worth it for $50. At the least, you could replace yours with his, giving you more VRAM for vram limited games, and crossfire for a lot more performance on other games that aren't being limited by your 1Gb.
It sucks paying for old tech, but $50 is much cheaper than a comparable card would cost new.
NOTE: It is called Crossfire with AMD cards.
EDIT: I noticed your motherboard will run the 2nd PCIe x16 slot in x4 mode if in crossfire. With microstutter issues, I would not go crossfire with that limitation, but if you are having issues with games stuttering due to Vram issues, you may find a 6950 2gb would be helpful, but it is old tech still, you could spend that $50 towards something new, but you'll have to spend closer to $200 for something worthwhile to replace it with. If not more.