|In most cases you can install only one ram module in any ram slot.|
" I tried installing the 1024 in the bottom slot and
the computer would not boot up."
If the "the guy at the computer store" DID NOT look up the ram that works for sure in your model, then it may NOT work properly in your mboard.
All the modules installed must be compatible with using them in the mboard and be compatible with being used together in the mboard.
See response 2:
Faster mhz ram will work in your mboard if it is otherwise compatible, but it can't run any faster than the max your mboard or cpu supports, and if it is NOT listed for your model you can't be sure it will work properly - it may work fine, it may NOT. E.g. sometimes Crucial lists faster ram than your mboard or cpu supports when they know it's otherwise compatible.
If you already have ram manufacturer's brand name modules, you can use the part number of the modules on the maker's web site to see if they're listed for your model (are 100% compatible with it).
"can I leave one 256 and install a 1024?"
If you have a mix of ram modules installed that have different max timing number specs / mhz speeds, the bios will automatically run all the ram installed at, max, the slowest of the max specs of all the modules installed.
If you have a mix of ram modules installed that have different ram voltages specified for them, the bios will automatically use the LOWEST ram voltage of all the specfied ram voltages for all the modules installed - that usually results in the ram modules that a higher ram voltage is specified for NOT working properly.