Gateway.....

Well I have an AMD64 gateway computer. Right now it has 512mb, not too good video card, 200 gig HD, 2 drives, floppy disk. I want to upgrade to 1 gig and x800xl or 6600GT without changing PSU. Will my PSU handle additional 512mb and x800xl/6800GT? I dont know what kind of PSU I have

You are going to have to find some kind of information about your PSU or this question is impossible to answer. I would venture to guess that you would be on the edge of not having enough power based on the fact that it is a Gateway.

I just found out its an ATX 300 watt power
supply

then probably not. the 12V line is probably very low and if you are going OC it using clockgen or anything as well, get something 400+W with at least 20-25A on the 12V line.

Fortron Blue storm, Antec Neo, OCZ 420/520, etc