If you're after speed you probably want two drives in a Raid 0 (striped) config - fairly sure the A8N will support SATA RAID. I have a pair of DiamondMax drives (mine in Raid 1 for protection) but if you can push your budget, the WD Raptor drives spin at 10K rpm (33% faster than most other drives).
Alternatively get a single 74Gb Raptor for OS and programs and use your existing 80Gb drive for data.
HTH
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Just checked MicroDirect and a single Raptor is £120 !
http://www.microdirect.co.uk/Product...46&GroupID=596