Well, first about the question of whether this machine will run Vista "Ultimate" with the high-end GUI as fast as XP. The answer is a resounding NO!!!! Why not? Each MS release only gets more bloated than the last, and increasing the eye candy will only further bog it down. That is more or less true of all software, but the "bloat rate" seems to be particularly high with MS software. When they recommend a 1GHz PIII and 1GB just to run the OS (XP rec'd a PII-233 and 128) that should give you a clue.
Okay, now about that computer.
1. CPU:
The 3.0GHz Xeons you chose (I assume that they are the 2M L2 90nm Irwindales) are not that fast. I built a workstation around a specific Supermicro dual-Xeon board and put two 2.8 Irwindales in it and it is not very fast. They are somewhat loud, but the loud HSF keeps the 1.12kg heatsink and CPUs relatively cool. I stuffed them into a regular ATX case and they run at about 37 C idle, and roughly 50-ish under load. That is not too bad, especially for 90nm NetBurst parts. If you can get 3.6 Xeons for $100 more than the about $500 you'll spend on the two 3.8s, by all means go for it. The 3.6s will run hotter, but getting more performance than two 3.6 Xeons will require getting something like a pair of Opteron 2.4GHz units or an X2 4600+/4800+. The price of the 4600+ is about what you'd spend on the wo Xeons and it would run cooler, so I guess it is dependent on what kind of a motherboard you want. I'd be willing to guess a 4600+ on a good 939 board will run a hundred or so less than the dual 3.6 Xeons on a decent dual-604 board, and the 4600+ would be equal or better to the Xeons in all but video and audio encoding.
2. RAM:
4GB should be fine. About every board will support at lest 4GB today, some will go 8, and dual-socket boards can do up to 16 in some cases.
3. GPU:
That 7800 should do you fine for the gaming and is severe overkill for real "workstation" purposes unless you do a lot of very graphics-intense work, and then it is just regular overkill. I put a cheapie $40 6200TC in my computer as it can drive my 20" DVI monitor more than sufficiently. If you don't need DVI or a dual-head setup, even integrated graphics can work for workstation purposes.
4. HDD:
I have a very similar setup. The 74GB raptor is very fast for its price but its heads are kind of loud. It's a good choice to use as a programs/OS disk. The 160 as a documents disk is fine, but I'd spring for a 250GB or 320GB unit as they are about $10-20 more than a 160GB and have a lot more space. The biggest size-for-dollar is right around 250-320GB right now, and that's what I recommend you go for.