I've seen benchmarks 1GB vs 2GB frame buffer and there's not much differences at lower resolutions so its not that important.
Any GTX560 would be a huge upgrade over a 330GT. The Ti is alittle better card but you'd be happy with either. Just make sure your PSU can handle it.
Check this review
http://www.tomshardware.com/reviews/...-top,2944.html