You can clean off the TIM with Isopropyl Alcohol. Stock cooling won't let you OC much if thats what you mean just watch your temps using HW Monitor or such.
As to your choice of aftermarket cooler, I personally use a Xigmatech, I did alot of research before picking it up. While the TRUE ((T)hermal(R)ight(U)ltra(E)xtreme)) Is pretty amazing when you lap both it and the CPU. I choose Xigma because I somewhat value the warrenty and didn't feel like lapping my parts. Also Many places , myself behind them now, Claim the temps are extremly low. I'm running 3.2 ghz with 1600 qFSB (3.6 Effectivly) and My temps idle at 35c load at 55c
Here's a pick of it in the case, and some links to a few of the reviews I looked at before picking it up, also, a link to egg with the item in question, 36 bucks!
In the cosmos1000 case. (Full Size case)
A lilttle gameplay but I sit nearly idle @ 38c with 3.2 ghz 1600 qFSB (400 x 8 Multi)
the 51c was from a 3dMark06 Benchtest, the heat is from the cpu test.
And the reviews: