Don't want to step on anyone's toes, but in the case of a card like the GT 630 the 4GB is only there to make it sell. Any game that could demand that much VRAM would have a GT series card limping about in the low single digit fps, and I haven't managed to find any situation where other software benefits from it either. I'm not saying it's a bad card or anything, just that the extra memory is pretty much there for decoration. I'd say even 1 GB is plenty for 1366x768.
Speaking of which, if I was the OP I'd consider figuring a new 1080p display into the upgrade budget. Not only gaming but also work, surfing and pretty much everything on a PC is infinitely more enjoyable with 2 times the screen real estate. If you subscribe to Newegg or TigerDirect's newsletters you'll find fairly nice 1080p displays for under $100 from time to time. If I had ~$200 to spend on an upgrade, I would take a 750 and 1080p monitor over a 760 and 720p monitor any day. Just my opinion.