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Are you sure of the memory amount? HP specifications indicate 1GB as standard:http://h10025.www1.hp.com/ewfrf/wc/documen...51389&lang=
The motherboard has dual channel memory architecture, so for optimum performance, DIMMs should be fitted in pairs, preferably matched, but at the very least of the exact same specification.http://h10025.www1.hp.com/ewfrf/wc/documen...cname=c00496280
If the system has a single 512M DIMM, it could be returned to spec with the addition of another identical 512M in the other channel, giving 1GB at greater bandwidth. If you add a 1GB DIMM to give 1.5GB, it would need to be fitted to the same channel as the existing DIMM, most boards will accept this sort of combination, some want them in a specific order of size.
The best performance would be achieved by fitting 2 matched 1GB DIMMS to give dual channel operation.