Folding@home PAH.. You want boinc! Seti, Einstein and Climate experiments, much more interesting than proteins.
As Billy said as long as you have an idea about computers its really simple. There are numerous overclocking guides on the net for xeons, the xtremesystems forum has a very in depth guide to overclocking 1.6lv's and there are a few good threads on the 2cpu forums as well.
Shame you didnt post this yesterday as i saw a nice pair of 1.6's on the auction site, almost bought them too! You should be able to get a pair for less than £50, don't spend any more. The two biggest expenses i had on my machine were the mobo, which is very difficult to find second hand, although there are a few websites that sell returned ones for slightly less, and the PSU which needs to be a single rail one with an 8 pin connector, adaptors are very difficult to find in this country. Other than that, just use what you have lying around, a cheep, cheep GFX card-if at all, any RAM, preferrably pc3200 to get the 200fsb. There are a lot of people using PC3200 value ram but some good ram may be worth it. And any old hard drive as long as it works and is reliable, unlike a certain other persons
.
You will also need a couple of HSF's, there are loads to choose from, Billy has the stock Intel Wind tunnels and I have a pair of thermaltake A1608 (can be picked up for £25 the pair. I had to remove the screws on one side of mine to get them to fit on the PCH-DL board though, hasn't caused any problems) There are also some passive versions that people are running.
Anyway hope thats helped a little.
PS:Can i put links to other forums?
i would hopefully have it on 24/7 so maybe use it as a file server as well.
as in 4Ghz per core? 
) as some PCH and PC-DL's have issues with clocking. The majority don't but there are about 40% that have problems.