Well, already got the 700 on the way, guess it's a bit of buyer's remorse kicking in making me doubt myself. This is all going into a Cooler Master 912 Plus, 200mm fans in the front and top. Other than the video card, not much will be in the way and I should have pretty good air flow. No overclocking as of now, but I wouldn't mind bumping things up just a little.
You should be alright!
I actually own two ModX 600W units = they've been great and one is over 2yrs old now.
The 700W version has received alot of mixed reviews.
The 500W unit is the best version out of the 3 imo.