hey, I ordered some g.skill HZ DDR500
I believe they'll be in later today!
What RAM you get will be dictated on if you want to OC or not. if you do not want to OC then a DDR400 kit of the OCZ or g.skill with tight timings would be ideal and cost-effective.
if you want to OC then RAM that uses Samsung UCCC is the most cost effective. (the g.skill I ordered, OCZ gold DDR500 = DDR500 gold GX, mushkin extreme performance DDR500, corsair ddr500pt) they typically will do DDR550 or so at 3-4-4-8 timings.
A slightly better solution would be some RAM with tighter timings that can do higher frequencies too.
the best you can get are probably some crucial ballistix tracers, but they're hard to find at the moment.
the next best and probably what I would have ordered if I had more cash are these: