No, Vista is no more secure than XP. The only difference is the blame. When you are hacked, well, you thought it was ok to agree to a clever pop-up that you thought was safe. Aw, you should have said no.
Upgrading ram on any OS to it's limits improves performance. So? Equivalent hardware yields superior performance on XP. Anything else is a salesman's lie.
Vista is just one step away from the ultimate safe OS. Rather than fix, they have blocked so many otherwise useful function in the name of "safety" that the next step to insure your security is to just flip the darn computer off.
There. Rest easy.