1. does the number matter if the lower # has better specs?

2. is this card considered a good price and if someone already knows about one thats better but about the same price please tell me.

3. This card says dual slot edition, does that mean that is plugs into 2 slots or takes up 2 slots? And if in the case that it does
 plug into two slots but the mother board is not sli, will I still be able to use it?

4. does the MBs in the card matter that much if there was a 512 MB and a 1 gig would it make much of a difference.

5. are radeon cards as good as Geforce cards? and what does this card compare to in radeon?

6. If the card says window certified does that mean that it can also run on xp?

1. Sometimes, for example 8800 GT is better than 9600 GT. Nvidia really screwed up the product naming scheme for their video cards.
In terms of performance, starting from the fastest part for Nvidia, it is: 280 GTX > 260 GTX  > 9800 GTX+ > 9800 GTX > 8800 GTS 512 MB > 9800 GT / 8800 GT > 9600 GT ...
ATi cards in terms of performance are as follows: HD 4870 > HD 4850 > HD3870 > HD 3850 ...

2. Not really, get a HD4850 if in that price range
3. Takes up 2 slots, means it will blow hot air from the card out of your PC case instead of contributing to the heat in your case.
4. At resolutions up  to 1680x1050, with medium AA, it does not make much difference.
5. For this generation, yes, HD 4850 is comparable to 9600 GT, but better.
6. Yes, look for the appropriate drivers on the vendor's website.