check the exact speed and cache latency of your current chips, get a 512MB of the same type and stick in in the spare slot = cheapest. No point getting faster 512Mb for the other slot. however, IF your board can take higher speed RAM (consult manual/web) getting 2x512MB that are faster and chucking the 256MB would be a tiny bit better and more L337, however, I bet if that RAM came with the machine it's running at full speed anyway, as is the case with most PCs.