Actually it makes more sense to use the 15k for the os due to the low seek\acces times, the 7200 rpm will manage the pagefile fine since 512 DDR is in the system and rarely does it use the pagefile as it is(few exceptions).
You need to look in the forums, for sale or trade, not hot deals, thats where the good stuff is.
As for serialATA its new=expensive, and as yet unproven in the market and not many mb's have it yet, and U160 moves over 35mb\s on average for me(44 with 10k drives average), plus U160 drives are quite cheap now if you know where to look.
I'm sure 4 years down the road, yeah serialATA and iSCSI\U320 will be common, but I dont have a trust fund to be on the cutting edge every 3 months or so, so I have to purchase wisely. Plus I can always grab a 64-bit card down the road cheap too.
The IDE drive I have now(ATA 100) only averages 23-25 mb\s at best, though I'm sure a WD 200GB drive would do loads better due to areal density of its platters.