newegg.com prices:
Athlon XP 2700+ $330 OEM, $342 retail
Pentium 4 2.8 $382 retail
not much of a price difference, but the AMD is cheaper. That p4 may outperform the AMD 2700+, but I expect that it won't be by much...not $40 worth of performance, anyway
And, personally, I like using AMD chips. So that's what I would go with if I were in your shoes.
As far as motherboards...personally, I'd go with one with the nforce2 chipset. The nforce2 boards allow locking of the AGP and PCI buses, making overclocking your CPU a breeze!
Second choice would be a Via chipset. The Via KT266 chipset will not support the 2700+ (I don't think it will, anyway). So you'd want a motherboard with Via's KT333 or KT400 chipset. Via chipsets are notorious for being unstable...but they have a mixed reputation. Some guys never have any problems.
There are several motherboards out there with either of these chipsets. You can go to newegg.com and shop the motherboards by what chipset they have.
After you decide what chipset you want the motherboard to have, it's really just a matter of picking the motherboard by name brand, price, and features...as motherboards with the same chipset will perform very closely.
good luck!