Getting a single 512 to add to your 256 would be perfect. 2x512 is a waste of money, since it costs twice as much but only gives you 25% more RAM. You may consider getting a 1 GB DIMM, but 256+512 MB is enough for almost everything. Later on, if you need more, you can then replace the 256 MB with a 1 GB and double your RAM.
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