With a 32bits OS, you won't be able to use more than ~3.2GB since part of the address space is used by memory-mapped IOs.
Unless you are going to upgrade to a 64bits OS, don't bother getting more than 1GB extra.
In principle, any DIMM should work. In practice, it does not always turn out that way. Check either your motherboard's QVL or your favorite DIMM manufacturer's compatibility check if you want to know which DIMMs are known to work with your specific motherboard if you are uncomfortable with picking at random or do not have a specific brand or model in mind.