S!
I think Steam support drops for those with XP and SP1 or lower so by looks of it XP with SP3 is still supported. Just my interpretation of it.
As of DirectX 9.0c support. ANY modern card that can run many games of today with reasonable FPS can run older DX versions. So kind of moot to always refer that so many have older rigs. Even a mid priced rig can beat the crap out of an old rig and upgrading to 64-bit OS for example is cheaper than buying a new one.
Matter of how you spend your money..for some a 90Eu/USD purchase of an OS seems expensive while they shell out the same in one evening at a bar or similar without any remorse..go figure
Maybe people should go Windows 7 64-bit more then..And luckily some games in near future will be 64-bit only. Time to drop off the XP and 32-bit. Never looked back after going W7 64-bit.