It all depends on your wants/needs. As far as gaming goes, a T7500 and an 8600GT would be better than a T7700 and an 8600GS. The ammount of memory the card has will not do anything for performance, unless of course it's less than 128 MB, but there is no difference in performance between 256MB and 512MB cards.
What I'm wondering is if that hard drive is enough for you? You said earlier that you want 200 gigs, so that would be something to think about... Otherwise, I'd take the Dell you listed over the HP.