It's possible to get higher than 4.5GHz, approximately 40% or so do get that level or higher. About 10% get to 4.8GHz overclock.
Yes, you need an aftermarket cooler. The stock Intel crap cooler won't hold temps down enough. However, you could get something like the $30 CoolerMaster Hyper 212 Plus and that's enough to get to your chip's maximum overclock.
I would suggest a different power supply, yes. Something from a quality manufacturer like Corsair, SeaSonic, or Thermaltake.
I would also suggest another 4GB stick of that Pioneer RAM. You're missing out on "free" performance with only one stick. Get it while RAM is still cheap -- who knows how long RAM will stay this low.
Unfortunately, the overclock won't help the slowest part of your system. That low-RPM hard drive will really hold you back, but only when loading apps/games and saving/moving/copying/etc. large files.