First things first: Get a new power supply. I learned that lesson the hard way when my motherboard fried
If your maxed at 1GB id reccomend a new motherboard because your motherboard will not detect new RAM
Corsair is my favorite kind of RAM but Kingston is another great brand. OCZ makes good RAM too. Check them out at newegg.com
As for the motherboard, you might need to install windows again which will kill all the data you have on your computer. Also, it needs to be compatible with all your other hardware. I'd recommend MicroStar, Intel, ATI, or Asus for motherboards. Newegg again has these motherboards.
Power Supplies are easy to install and have no drivers so you should do that first.
Antec makes awesome power supplies, but ive had good luck with my Rosewill 300 Watt for $29.95USD
If you need help, I can help you install everything, for ive done it all myself before.
Just gimme a holler
Good Luck,
sw123