Do you mean ATA or SATA, the whole "ATA serial" thing kinda throws me off in regards to what you are talking about.
But anyways, all new motherboards now should support both ATA-133 and Serial ATA, so that won't be a problem at all regardless of what you have.
In regards to the data transfer issue, you can invest in a network card for each computer and a router. You can set up your new PC as a network server for the other PC and set the other PC to boot from the network.
This means you can literally keep the drive on your media computer empty and easily access all files from your other computer without having to move any data around. You simply have to keep your server computer on when using the other one, and you are set.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w