Go with the 2x2Gig memory.
You can stay with 32 bit Operating system (Will have approx 3.2 Gigs of memory for OP & Programs) works great in my system. Did not see any performace hit. Or, as previously stated you can go with 64 bit Vista and get full 4 gigs. Myself I'm happy with 4 gigs in both Vista32 and XP pro 32 Bit (Have dual Boot).
Added - 2 x2 Gigs was no different to OC than my 2x1 Gig sticks (4 by 1 Gig sticks required raising the FSB & (G)MCH voltage by +0.1 V for stable operation which I backed off when converting to the 2x2 sticks.