Only official differences between the KT133 and KT133A I've seen so far are the 133MHz bus support. Sight unseen at this point, yes the KT133A would be better for future upgrades. But IMHO I don't believe it's a worthwhile upgrade from a functional, well-featured KT133 (like the Abit KT7, Asus A7V, MSI K7T Pro2 or Epox 8KTA+/2) for many users. If you're getting a new system, I'd seriously consider waiting the month (or even less) for the KT133A, or possibly the DDR system boards. I'd also expect that fairly soon after its introduction, most all manufacturers will reintroduce/replace their KT133 products with KT133A boards.
OT, how do you like your Aztech/Philips Thunderbird sound card? I've been mulling over Philips' new Acoustic Edge sound card (to be released real soon) and the Seismic Edge (which is $39 after rebates this week at Circuit City). How's the sound quality and QSound?