I have these speakers, and just so you know, you do NOT have to plug these in using the spdif connector. You can plug in an analog signal just fine, I assure you.
As for the 735 vs. 635: They are exactly the same. The 735 is the Gateway model # (gateway was the primary buyer of this line). But regardless of what it says, I promise you that you can plug this in using hte analog connector and it will work great.
You should check ebay, however. It's flooded with these, and many under $20.
I agree that these are incredibly solid speakers. I was very impressed with the sound.