I was wondering: If I took a 64Gb Crucial M4 and SRT'd it with a 500Gb Seagate, would I still be able to throw a terabyte drive in as a second drive letter and be able to back up to it as if I just had two HDDs?
Yes. Your Seagate and M4 would be your C: drive and your Terabyte drive would be your D: drive.
Second but slightly related question: Does it matter if I RAID 0 or RAID 1 the SRT setup? I'm planning on RAID 0.
You do not create a RAID array with Intel SRT. The ports your Seagate and M4 are connected to have to be in RAID mode