You can get in on the beta if you own either an ATi HL2 voucher, or CS:CZ.
I haven't read most of this thread, but I'd probably return the GT, sell the XTPE for a large profit, and look for a $350 BFG 6800OC @ Outpost, or preorder a $333 XFX 6800GT at Buy.com.

If you want a card now, though, I'd have to say stick with the GT if you can sell the XTPE and reap the huge profits others seem to be enjoying. If you don't care about making a quick, capitalist buck, then I'd take an XTPE for $65 more than a GT. I'll take the hit in Doom 3 if it means better framerates in most other cases. VIVO is a nice bonus, too.
Edit: I see you paid tax on the GT, which makes the XTPE only like $30 more, no? That changes my suggestions slightly:
1. Remains the same.
2. In addition to eBaying the XTPE for huge bucks, I'd also just return the GT and buy one from NewEgg to save ~$30 on tax, or get in line at Buy.com and save $100 overall (~$430 -> $333).
3. $30 more for a XTPE is about as close to a no-brainer as possible.
jim, firstly, I don't think a child will Paypal him teh $600!!1 for a video card on eBay. Secondly, no one who's shopping for $600+ video cards on eBay is hurting for cash, be he child or adult. As for charging as much as ppl are willing to pay, I'm probably with Hank Reardon [sic? it's been a while since I ploughed through Atlas Flinched] on this. He wouldn't be lying to anyone, like with an OC'ing RMA. He's just offering the card for $600+ for whoever wants it.