I have a new custom system with 1 gig of ram and I just noticed that there are no more slots available-there are 2- 500 sticks now being used-I want another gig-where would it go?-are there expansions for ram that would allow another gig to be installed-this is new equipment and I don't want to throw out the ram in there to replace more-any ideas?

First confirm that your motherboard will support 2 GB of RAM.
Unfortunately, if you want more you will have to replace the modules you now have with larger ones. As far as I know, there is no way to add more expansion slots without getting a new motherboard.
The good news is that you could probably sell the old RAM modules to someone else.

Go to Newegg.com or Zipzoomfly.com and look under memory. You are probably using DDR ram with 184 pins. You can purchase a 2 GB pack containing 2 1 GB sticks for about $200. This would be dual channel ram so if your new custom PC has hyperthreading you will get really good responce.
I build custom PC's and install SOHO networks on a limited basis. My test machine for trying new products has a P4 3.2 Ghz CPU with 2 MB cache and 4 GB DDR400 dual channel ram. I do a lot of digital photo work like converting from RAW to TIFF and this works fast.
Good luck, Fred