I think the 3870 is built better. I am not a fan of build of the 8800 (G92), especially the GT. It looks like a budget card from 3 years ago. I decided to go with two Asus 9600GT Silent, since I really wasn't thrilled with any of the high end cards. I think the ATI looks like a beefy card. I know looks shouldn't be that big of an issue, but it is to me. It seems the new 88s are not built to last like the old ones. The new GTS not bad, the GT is pretty flimsy. I also installed on in my nephews computer with identical components to mine except card. I had the old 8800GTS (G80). His doesn't outperform mine at least not in Crysis. In fact I think my numbers are better a higher res. If I were to go with an 8800GT, it would be the egva Akimbo. The Akimbo costs more but has an excellent cooling system and overclocks really well. Or do like I did and get a 9600gt save your money for the next gen.
The 3870 is definately not built better. It suffers from major bottlenecking issues. How did they manage to get a gpu with so such high floating point capacity and raw processing power to perform so poorly? It's such a shame. The 4870, with twice the TMUs, should ease this bottleneck, and we may finally get to see what it's supposed to do.
In performance, 3870 is identical to 9600gt, and slower than 8800gt and g92 8800gts. g92 8800 gts is the best bang for the buck right now at $169.99 after MIR.http://www.newegg.com/Product/Product.aspx?Item=N82E16814134037
Ideally, wait for 4870 or possibly 9900.