Liquid cooling is not necessary for a Quad core CPU, Only if you plan on OC'ing as other enthusiasts do. I just believe myself that liquid cooling is an accident waiting to happen where you are about to lose your entire pc if the system stops flowing or breaks down.
Personally, I would go with a nice air cooled case which is cooler then liquid cooling in some cases. Go for a zalman or any other good quality brand, read the reviews, but some people are stupid when they put up bad reviews, ie;("The copper plate on the bottom is bent!, this cooler sucks!"), usually thats designated on the user not spreading the thermal compound the correct way, failure to do this can actually fluctuate your temperatures 10 degrees higher.
As for the AMD phenoms, go with Intel's quad core, which Stomps the phenom out of the way right now, give AMD a few months to square out the problems. I used to be an AMD fan myself and frankly I don't like the way things are going with them. A bit flaky imo.
As for the PSU wattage, I wouldnt go less then 550-600, A good price-equivalent power supply would be an OCZ gamexstream, Any of the antec series, and 'pc power & cooling' also makes very good quality power supplies. There are a few things to remember when looking at a power supply, The first is, look how many amps are on each rail, it is best to get less rails with more amps, such as one single rail with 52 amps is amazing. OCZ uses quad rails with medium range amperage, which i think holds up pretty good, I have a system reccommended at 720 watts, but im using a 600w power supply on a budget, but it holds up fine, because it's made of quality.
OC'ing a CPU all depends on the motherboard, Without the proper OC'ing knowledge, you can fry your chip if you go too high.. google it or contact me personally and i can help you out.
As for the RAM, go with the 4 gb gskill if you're really looking for a budget, those are nice for 100$, crazy stuff.
Usually, I pick ram for quality, but since your on a budget that ram has a 5 egg rating at that cheap price, go for it.
Contact me personally once again for any other questions or i can continue in this thread.