OK I've had my 7800GT for a couple of days now, and here are my impression of it (coming from an X850XT PE @ 600/620): the NV card is about 20% faster than the ATI card, give or take (more in FEAR, less in Far Cry + HL2). Far Cry performs about the same, but you now have the option of running HDR, which does kill performance (expect about a 50% drop), but still runs fine up to about 1600X1200 (some big outdoor levels do chug with HDR, but considering you can't do HDR on the ATI card at all, it's a bonus).
Half Life 2 runs about the same; 1680X1050 runs great for me in regular HL2 with AA/AF, and is a tad choppy in Lost Coast (with 2xAA/8xAF), just like the X850XT.
Multi-monitor support is similar; load temperatures are similar; 2d quality is similar. If you're using stock cooling on both, the X850XT PE is noisier @ high fan speed than the 7800GT; unfortunately, the GT pumps up your case temperatures a fair bit since it blows into the case, not out of it like the dual-slot X850XT. I recommend getting an NV silencer.
FEAR runs much better than on the X850XT PE (although I sold my card before the "rename FEAR.EXE" fix came out. Nevertheless, I don't think it would improve high-res performance that much). At 1680X1050, I was getting 32fps average, with dips into the teens on the X850; with the 7800GT, I run at 48fps average, with a minimum of 30fps.