[size=2][font=Comic Sans Ms]OK...I've tried to get the answer to this for over a year, and NO one seems to know. I have a HP Pavillion XE783 Desktop...it came with 64 ram....after determining (online) that i needed pc100 ram, to up grade (512 max) I went to circuit city, told him i wanted 2 sticks of 256 pc100 ram, but he gave me pc 133 ram and said it should work, get home install them and pc wont detect but ONE stick of new ram...tried putting them in different slots, left one in and added my old 64...nothing...it will only detect the one 256...so at least it was better than 64 ram I had and it functioned well, no problems..so i was satisfied. My question is...since my HP will detect the 256 pc133 ram but calls for pc100....can I just get ONE stick of 512 pc133 and replace the 256 stick, will my pc then detect the 512?

Referring to this page -
HP Support
- under memory it says the maximum supported is 512MB (two of 256MB) so you should be okay with what you bought. PC133 is fine for PC100 machines, it just means the RAM won't run as fast as it could in another machine.
Do both sticks work on their own? i.e. you don't have a defective stick? If so maybe you have a defective memory slot. Buy a can of compressed air and blow out any dirt from the socket. See this tutorial for more info - Cleaning the Interior of your PC

As to your question about a single 512MB stick - No, I don't think that's possible.

hth

Yes each stick worked on its own...BUT it wont detect but one...256 NOT 512, so I removed one of the sticks..and my pc worked fine much faster than with just the 64 I had...so are u saying ONE stick of pc133 512 will work?

You may want to check and see if HP has a BIOS upgrade for that computer. That may be what is limiting your RAM.
FYI: You computer will only use low density RAM. If you use high density RAM it will not reconize it. Lowdensity PC133 is backwards compatible. It can be identified by having 8 small chips on each side of the stick of RAM. High capasity PC133 will only have 8 chips on one side.