The 550ti is fine for gaming at up to 1080p with high (not ultimate) settings. The 550 non ti falls a little short IMO. And you only need 1 GB VRAM for 1080p gaming. No need to spend extra money on a 2GB card.
As for system memory, if your using 32-bit windows, 4GB will be fine. If you are using 64-bit windows, the extra 4 GB will come in handy.