The GTX 550 Ti is a pretty decent card. When you are talking about comparing that to the 560 or 570, you are only talking about marginal improvements.
12GB should be sufficient for most projects.
I understand that the i5 does NOT support hyperthreading. That might be a constraint if the process you are running is CPU bound and not GPU bound.
I think you may want to check"maximum quality" to force the GPU use? I can't quite recall. seacrh for the correct term on here. Also lots of good info on GPU comp[arisons:
http://www.studio1productions.com/Articles/PremiereCS5.htm
Information here on memory usage. You can see how they monitor memory usage toward the bottom. You might also run these benchmarks and compare your machine to see if anything seems out of whack:
http://ppbm5.com/Background.html
http://ppbm5.com/DB-PPBM5-2.php
Good luck! I understand the frustration. Just use the energy to try to move you forward! Good luck!
Michael