Computer Build Help

I am building a new PC and wanted to ask if there were better/cheaper parts I could use.

Motherboard:
GIGABYTE GA-Z68X-UD3H-B3 Intel Z68 ($160)

CPU:
Intel Core i5-2500 Quad-Core ($210)

RAM:
Keeping the same one from my old build. Has 8 GB's (4x2GB each) Should I upgrade it to a 2x4GB each or does it not matter?

Video Card:
EVGA GeForce GTX 550 Ti ($115)

Hard Drive:
Keeping same one from my old build as well. Has 800 GB or more.

Case:
NZXT M59 Gaming Mid-Tower Case ($50)

Power Supply:
Not sure whether to get the Cooler Master eXtreme 500 Watt ($35) or the Corsair CX600 600-Watt ($70). Did some reading up online and I read that the power supply needed is usually the biggest thing new builders screw up (whether it being getting too much or too little power). SO wanted to ask if 500 would be plenty for this build or if I should get the 600 watt one.

My CD/DVD drive and sound card are nothing special so those shouldn't take up a lot or power.

The Monitor I have is an LG Flatron (about 4 years old) and I wanted to up it to a 22-24 inch screen. I've been checking out Asus and BenQ monitors and the ASUS VS247H-P 23.6-Inch LCD Monitor or the BenQ RL2450H (I just picked these 2 out but I had a whole list of monitors on my old computer but since the graphic card crapped out on me I can't access my files on the hard drive till I build this new one). Could really use help picking a monitor though because there are so many different types (TN, VA, LED, LCD, etc...). Budget for monitor is about $250. Could go up to 300 but that's the stopping point. Using the computer for gaming and video's and everyday use. Would like it to be at least 22 inches or more.

So those are the parts for what I'm building. Are there better parts that I could get for cheaper or anything like that or from different places for less? Those are all from Tigerdirect.