oo, had you already said that you have the gpu and os? i dont recall seeing that, lol.
ok, then for the ram, just get the ram thats in the combo without the gpu, it should save you a bit more money.
about the OS's, yes you can have two on the same hdd, but the win 7 rc is much better by far than any verison of vista including ultimate. if you really want to use vista ultimate, then you can install that, and install win 7 on a seperate partiton if you want to "try before you buy" in a sense.
but im pretty sure that you will not want to go back to vista once you use 7
edit: thx Trotter