Help Purchasing New RAM

These boards have been awesome with helping me so I am coming back for me, I would like to replace the RAM in my system with better high performance RAM.

HP - Compaq Media Center m7580n Desktop/PC .

Crucial Memory Scan says:
Each memory slot can hold DDR2 PC2-4200,DDR2 PC2-5300 with a maximum of 1GB per slot.*

Does this mean I can not use higher performance, or can someone help explain what I can use, I am upgrading the PSU and Video Card as well.

HD Radeon HD 3850 - http://www.newegg.com/Product/Product.asp?Item=N82E16814129101
Earthwatts 480W - http://www.newegg.com/Product/Product.asp?Item=N82E16817371006

Any advice on what to get would be helpful, would like to stay under $200 USD

Thanks
BlackFireFox