The Momentus XT is a very impressive hybrid SSD / HD hybrid ......http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/39621-seagate-momentus-xt-500gb-hybrid-hard-drive-review-12.html
The performance numbers we saw in testing were interesting to say the least since synthetic tests really don’t do this drive any justice. Where it really takes a step away from the pack is in the all-important real world tests. In every application we tested, Seagate’s hybrid drive simply demolished the competition.
But the SSD portion is only 4GB and if ya regularly use large files......that means it's just a normal HD beyond 4GB. The newer 750GB model has 8GB which is more well suited to today's usage.....but two of the 500's in RAID 0 would certainly rock for quite a decent price.....assuming that's enough space for you.
Here's a review of the 2nd generation 750 GB modelhttp://benchmarkreviews.com/index.php?option=com_content&task=view&id=806&Itemid=60&limit=1&limitstart=9
At the point that these things hit 32 or meybe 64 GB for the SSD portion, methinks having separate SSD/HD could become passe.