I'm building a computer with these parts:
CPU:Intel Core 2 Duo E8600 Wolfdale 3.33GHz
Ram: CORSAIR 4GB (2X2GB)
Video Card: Diamond Radeon HD 4850 512MB
Hard Drive: Seagate Barracuda 550 GB
Motherboard: Asus P5Q-E Green Motherboard - Intel P45
Case: NZXT Lexa Blackline ATX

I really dont think I need to do any serious Overclocking, but if I did in the future would this be a good system to do it to?

Way good. It will do 4.5+ on a good air cooler easy!

I'd switch to NZXT Tempest. Similar price, but better cooling.
http://www.newegg.com/Product/Prod [...] 6811146047

Get a 750TX PSU, it will let you add a second video card later when/if you need it. If that sounds interesting to you, also change the MB to P5E Deluxe X48, it's better for two cards. If not, a 650TX PSU is enough.

You don't need a cooler for now, but when you do decide to overclock get a HDT-S1283.

I don't think the 4850 is an ideal choice in an OCed system because of the way it cools. It just dumps it's heat inside the case.

While part of a 4870s heat does still end up inside, it has the double slot cooler, vented out the back.

Ideally I think a case should have 4 120mm fans.
Two in front on the lower half of the case to draw air in. They should not be overly obstructed by drive cages.
One in the back venting out, up high by the CPU. The last one on top in the back, venting out.
In this scenario the PSU is on the bottom.

Added vents in the side etc., are not really desired.

There are other solutions too, I just think that is the best.