DDR2 mode ~ combining a 512mb RAM card with a 1gb, without dual mode what dose this mean?

This means that since the two ram sticks are not "matched" you system will not operate in "Duel" mode. And yes, this can cause a small slowdown. To maintain the "duel" mode the sticks must come from the same manufacturer, be of the same size and speed, and in most cases, have the same case latency. If you still wish to run the two unmatched sticks that you have, I would run the larger of the two in the primary slot and the other in the secondary slot. Note: Having two matched sticks will double the speed of the systems front side buss.

Bare this in mind. There is a huge difference in the ram you can purchase. Many can be incompatible and cause some really strange behavior from your computer. There are many different manufactures of ram out there and many are considered to be cheap or incompatible. Always buy top of the line when it comes to ram this will guarantee trouble free operation. Here is the best you can buy, let them scan your computer, they will tell you what you need. http://www.crucial.com/ One last thing also to remember there are many schools of thought on this subject that I happen to agree with. Always use matching ram 2-512's or 2-1gig sticks don't mix and match like a 512 and a 1gig even though this will sometimes work but more problems have been reported doing this.