clutchc :
+1^
The GTX 750 will be a bit faster, but the extra VRAM will be more beneficial if you game at 1080p and like to use some Anti-aliasing and/or higher textures.
Most GTX750's have only 1GB of VRAM. All of the GTX750Ti cards have 2GB. In North America the 750's that have 2GB cost almost as much as some of the 750Ti cards.
*If the ONLY choice is the GTX750 1GB versus the 650 2GB then get the former. The GTX750 has a better GPU which is more important at this performance level than having more than 1GB of memory.
Other:
If you have only 32-bit Windows, don't get more than 1GB of video memory.
Performance Summary:
http://www.techpowerup.com/reviews/ASUS/GTX_750_Ti_OC/25.html
*The GTX650 is roughly the same performance as the HD7750.
http://www.techpowerup.com/reviews/MSI/GTX_650_Power_Edition/27.html
(So the GTX750Ti is 2X faster than a GTX650 for reference. Assuming no CPU bottleneck in the game. The GTX750Ti is roughly 15% to 20% faster than a GTX750.)