Here are some examples of water cooling in an Antec 900:
http://i290.photobucket.com/al...62/Grieve666/good1.jpg
http://www.dropshots.com/steeda1974
http://www.techpowerup.com/gallery/1038.html
http://www.radiical.com.au/pro...Antec900ProjectTwo.asp
The general consensus from what I've read on the 'Net is that the 900 is not well-suited for water cooling (although it can be done with some ingenuity). It's great for air cooling though.
Edit:
Your biggest challenge is finding a place for the rad since its the biggest thing in the loop. You must take into account the thickness of at least one fan (or two if you go with a push-pull config) when working out where it should be placed inside or outside the case. Quite a few people stick the rad on the rear of the 900 using a rad box.
The water block doesn't matter since they are all roughly about the same size.
For a pump, you can use a Swiftech MCP350, 355 or 655. Replace the 350 or 355 pump top with an XSPC top, XSPC top / res or EK top / res, so that you can use G1/4 1/2" OD barbs.
The res can be a bay res (if you have a free 5.25" bay or two), the Swiftech Microres or a XSPC top / res (used on the MCP350 or 355). You could also use one of
these, which has the combined res and top (similar to the XSPC top / res). Or you could use
this res. Note that the latter two reservoirs come in various sizes.
Note that you may get a whooshing noise in the res if you use a bay res and it isn't completely filled (Aigo stated this). Be warned and choose carefully if you go down this route.
You may choose not to bother with a res and just use a T-line instead. This makes it more difficult to bleed the loop (it takes longer). You could always use a T-line and a res; the T-line can then be used to drain the system.
If you are using the Swiftech MCP350 or 355 with 1/2" barbs and the res you choose doesn't have a combined top, then you will need to use the XSPC top on the pump (as I mentioned previously).
You need to make sure that your pump is matched to your water block(s).
As masteryoda34 said, use 7/16" or 1/2" ID tubing with 1/2" OD G1/4 barbs. Woodbutcher uses 3/8" ID tubing on 1/2" OD barbs because he like the 'clean' look of the narrower tubing. However, this makes it very difficult to get the tubing on to the barbs due to the differences between the ID (tubing) and OD (barbs).
Note that nearly all the fittings used in water cooling are G1/4. There are some oddities, but for the most part they will be G1/4.
You can use cable ties or worm clamps to keep the tubing on the barbs.