You've answered your own question. You want DX10/11, therefore you need to upgrade, and since Vista is inferior to W7 then W7 is the way to go. 64bit is compatable with the vast majority of software (exceptions being those with 16bit code), and you already have 4gb of RAM so some is wasted if you go 32bit. 6gb is overkill at the moment IMO.
Untutored carnage is useless in the face of an educated bullet
The penalty for masturbation in Indonesia is decapitation