so my sister has this computer and its kinda old she has 1.00 GB Single-Channel DDR2 @ 266MHz (4-4-4-12) can i buy an 800MHZ ram to upgrade her's. And if not what the aftermath would be
Check out the vendors website to know what memory is compatible. Also check what the maximum amount or RAM is for the system. DDR2 is also a lot more expensive than ddr3 so if you planned on adding more than 4GB, you'd be better just grabbing a new MB/CPU/RAM combo for nearly the same price using DDR3.
If you are using DDR2 you can really use any DDR2. I'm assuming you are actually using 533mhz memory since there is no 266mhz memory (called pc2 4200).
I would not put 800 in however since if it came with 266 i'm sure the board won't make use of the speed and its extra cost. Just go pick up whatever ram is the cheapest with the best low speed timings.