building my first computer. please help :)

alright, no barebone kit then. i'm new to all this. just thought it'd be easier, but i'd rather have better quality than cheaper or easier. thanks.

Well since you have a video card already, that's a huge decision already made (if your a gamer).

I see I'll be the first to tell you this, but you can build yourself an nice C2D system for the budget you have, and it will give you more upgrade options down the road. If you are into overclocking then the C2D is an even better choice. Lot's of people here will steer you towards C2D, I'm not going to be as militant as some, it's your money to spend as you wish. Just do yourself a favor and look at Tom's CPU comparison chart. A little research can help to avoid regret and disappointment later on.

If I had that budget, I would be looking at a E4300/E6300/E6400 for a CPU. Next I would consider a P965 chipset equipped motherboard (P5B maybe), either PC2 5333 or PC2 6400 (depending on how high you want to overclock, if no OC, just get PC2 4200)in a 2 X 1GB kit, and a 500-600+ watt PSU from a reputable manufacturer. Casewise, that's more of a subjective and personal choice, just pick something that has good airflow (the Antec 900 does). Of course you'll want to ditch the stock HSF on the CPU if you are going to overclock. Your choices here will be somewhat dictated by the case and motherboard you choose. If it comes down to this you'll get plenty of advice in this area.

If you decide to go down this road, plenty of people (myself included) will have plenty of advice on particular models/brands of everything you'll need.

Geil ram would be a real kick ass part for your computer.
As far as the your CPU, I would recommend an Opteron 165-170. It delivers high quality performance due to the high purity of Silicone found in the chip, and its cheap. If you can still return your X2, I would definetly do it.
as for ram, try and see if you can Find Geil EVO1.
This is supposed to be a type of hybrid ram.