For 2 512 meg sticks, you may have to sell them; look for a kit of 2x1 gb or 2x2 gb used on craigslist if possible. Use name brand only: crucial, kingston, corsair, samsung (hynix), micron, pny, or ocz. The first three are preferred. Use cpuz to see if you have pc5300 (667 speed) or pc6400 (800 speed). These will be read as 333 or 400 by cpuz (half their actual speed).