Ninja Edit: This article
explains the difference pretty clearly: The GTX650 is an overclocked GT640, the 650Ti is a cut-down GTX660.
Not sure I'd say the 650's an overclocked 640, per se - the main differentiating factor comes from the 650's use of GDDR5, versus the 640's DDR3. There's a chasm of difference in the resulting bandwidth, and the 650's much better-equipped to handle demanding rendering scenarios as a result.
As for the rest? Yeah, there's probably not much reason to spring for 2 gigs of VRAM on the 650 unless you plan to work with some very large data sets...
Work: FX-8320, GeForce 750 Ti, 16 gigs RAM, Lubuntu 14.04 x64
Play: i5 3570k, GeForce 660 Ti, 12 gigs RAM, Windows 8.1 x64
HTPC: Athlon X4 750k, GeForce 650, 8 gigs RAM, Windows 7 x64
WTF: Celeron E1500, GeForce 550 Ti, 3 gigs RAM, Slackware 14.1