Funny, I picked up an Xbox360 Elite less than a month ago as well. Anyways, OCZ makes a good 700W PSU for $150 ($140 after MIR)
http://www.newegg.com/Product/Product.aspx?Item=N82E16817341002
I use it and haven't had any problems with it other than it's size; it's too small for my full tower so I had to to a bit of modding to make it fit. It powers my dinosaur of a system without issues:
AMD Athlon X2 4800+ @ 2.6GHz
2GB PC3200 DDR (2-3-3-6)
eVga Geforce 8800GTX
eVga Geforce 8600GT
Creative X-Fi Xtreme Music
BFG AGEIA PhysX
PowerColor ATI Theater Pro 550 PCIe 1x
250GB Western Digital 7200RPM 16MB cache SATA HDD
3 DVD/RW drives
1 Floppy
4 80mm LED cooling fans and 1 120mm LED fan.
LED backlit fan controller