gt240 is better but it supports directx 10. 6450 supports dx11 but is weaker than gt240.
none of them are good for gaming (in traditional sense).
they're both good for web browsing and basic use such as office document viewing and editing, movie watching etc.
1 gb vram is preferable now. if one uses multi monitor or vram intensive tasks, higher vram helps. even with same vram, the gt240 would be stronger. check this gaming gfx card hierarchy chart -http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html