I'm finding myself in the same situation. Personally I would suggest going for 1GB total right now. The problem I've had is seeking out what ASUS recommends. I looked at their list of 512MB modules, and which ones could handle a maximum of 4 DIMMs in the motherboard. I couldn't find those precise chips anywhere. If anyone knows, please report in. I was thinking of Corsair TWINX1024-3200C2. The data sheet at
www.corsairmemory.com states tested and packaged in pairs..modules tested together in ASIS P4C800 or A7N8X test setup. I also see Corsair has a TWINX1024-4000 that was similiar but it's quite expensive... around $390. I went to
www.essencompu.com which I thought was an overall decent site due to the detail provided, recommendations, ratings, etc... but the memory they listed as a suggestion when looking at the P4C800-E deluxe says it won't work for i875P motherboards. Their support/sales/service doesn't have any comment on that.