Well the 3870 has onboard 5.1 or 7.1 sound through HDMI if you can use it at all. Plus, they just released new Catalyst drivers that improve the performance of the 3870 in an ass-load of games. Between the 8800GT and the 3870, I'd go with whichever you can find cheaper. Then again, if you already have an Nvidia chipset why not stick with the 8800GT. If you have an AMD or ATI chipset, I'd stick with the 3870. I personally like the SAPPHIRE Toxic 100225TXSR from Newegg. It's currently $149.99 after a $20 MIR and $10 off with code EMCAFCJAD. But again, I'd go with which ever is cheaper between the 8800GT and HD3870 and depending on your current motherboard's chipset.
--Arson94