Although the Zalman Reserator 2 is very quiet its not that powerful.
It's good enough to cool a single Nvidia 8800GTX card but the hotter ATI HD2900XT plus a pair of Xeon 5160 3ghz 80w processors would overload a single Reserator 2.
You'd probably need to consider a pair of Reserator 2s: one for the graphics card and then one for the two processors.
If you're going to fit quad core Xeons soon you may find it necessary to use a reserator 2 for each CPU though.
That could work, although the next problem is that Zalman don't make waterblocks for the Intel Xeon 5160 Socket 771 CPUs or the ATI HD 2900XT graphics card.
The Zalman Reserator 2 is made from aluminium, so you really want to use aluminium waterblocks too to minimise the risk of galvanic corrosion.
If you added copper waterblocks (eg. Danger Den products) you could potentially run into problems over time.
Anti-corrosion additive could slow this enough to make it workable but it's something to bear in mind.
It would probably be better to use an active water cooling setup instead with some fans on the radiator.
For checking the power draw of your PC something such as a kill-a-watt power meter will tell you how much power your PC is using.:
http://www.the-gadgeteer.com/review/kil ... tor_review