Make sure that you get Vista 64 bit, otherwise you cannot really use the additional memory beyond 2 GiB!
The vendor of my machine pre-installed the OS and took 32 bit while offering "64 bit technology" - and won't exchange the OS. I tried to talk to M$, but those are only robots repeating over and over that I need that vendor to exchange it without listening that that vendor sells 32bit as 64bit and doesn't exchange it. *GRRR* At least they should put pressure on such companies or stop them from selling their products!
We only use DX10 for tech demos of emerging technologies - so we'll have one or two machines (we're buying four of these) with dual boot XP64/Vista so we can demo DX10 - we currently use four machines (desktop) networked together - two running XP, two running Vista - playing S.T.A.L.K.E.R. - Clear Skies to highlight the diffs between DX9 and DX10
Be careful with Dell though, they can be a bit machiavellian as far as support goes. Products are generally good, but I found they were built to fail after a certain lifespan.
This is just my experience, but be careful with the power connector...