If your current card is fine for now, then there isn't much point buying a new card right away. If you do want to upgrade, and somehow plan to recover what you spent on the 550, then I would say you need a 6870, or 5870 for a meaningful upgrade. A GTX 460 or Radeon 6850 would certainly provide a meaningful boost over your current card, but not a great one.
personally I'd bump up a little more to a gtx 560 ti for 235 but then again we don't even know what resolution the OP is gaming at (and for that matter, for 30 or so more dollars, the op could go to a 6950 2 gig and flash it to 6970 but that's getting ahead of ourselves a bit)
LFD2 (will run fine on a lower end pc), witcher 2 and civ 5 are not the most demanding games (of a gpu) I believe the civ 5 is more reliant on cpu power but all in all the OP could do much better (imo) for the money then a gtx 550, which is a fodder card.
wrong...460's kick sooo much a$$ that nvidia wont even license them for triple or quad sli....it would cut too hard into their 580 sales and it 300+ for a 2gig 560ti so.........2x 2gig 460's=best bang for $$$$handsdown
why would you want more than 1 GB versions of any card, unless you're running really high resolution and multiple monitors?! it's much better to stick with 1 beasty card than bothering with SLI/Xfire.... it seems like you're a big fan of 460's =)))