Thanks for the suggestion Tecmo. That's a good heatsink, but you may want to also check out the Sunbeam CCF- sorry couldn't get a link because Newegg is acting very weird, anyone else experiencing this?
If your new to this, overclocking the x4 620 might not be the best idea(although it is still very doable with research), as it is not a black edition
http://www.overclockersclub.com/reviews/athlon2_620/3.htm
Read this article! If you want to give it a go. The x2 550 is still a very viable option as it has the better gaming performanceand is more overclockable then the x4 620(due to it being a black edition) but you sacrifice 2 cores for some L3 cache.
Anyways both are decent, with the Hyper 212 getting better reviews, and the CCF being a bit cheaper w/ the $15 rebate. I have not actually had either so if someone with personal experience could help him out? It seems like the hyper 212 is the safer bet though.
Remember if you going with the Hyper to get some thermal paste like Artic silver 5 or TX-2.
And yes the Antec 300 illusion has more than adequate cooling, some $100 cases have worse cooling.
Let me Know how the build goes.