My usual modus operandi is to use the largest DIMMs available (1GB in your case) so as to leave empty slots for expansion. However if your computer really does have a maximum of 1GB it may be moot.
Check your manual carefully - it may be that 512MB is the largest single DIMM it accepts and therefore a single 1GB DIMM may not work.
However it's often the case that the manual is written for the largest memory available at the time. For example, it may be that the only reason your documentation indicates that the limit is 1GB is because the largest memory available when your computer was sold was 512MB DIMMs, and there's only room for two of them. But newer, larger DIMMs could possibly work fine.
If you can give the model number of your computer or motherboard, someone who's familiar with it may be able to give a more definitive answer.