An answer from Tom's Hardware... LINK.
I have run the same 1000 watt ULTRA (modular) for over 4 years, with many different configurations and with out missing a heartbeat. Here is my latest power hungry system:
EVGA X58 3-way classified
Core I7 920 (OC'd to 3.2Ghz)
6Gb OCZ Tri-channel RAM (ddr3 @ 1600Mhz) and a "crab cooler" on top of the ram sticks.
TRUE 120 Air-cooler (changing to Swiftech H20-220 Ultima XT CPU liquid cooling kit) this weekend.
Coolermaster HAF 932 full tower case.
2 LG DVD burners
NZXT Sentry LX Dual Bay Fan Controller
2 x ASUS Radeon 5850 Graphics in crossfire
Asus Xonar Essence ST 24-bit 192KHz PCI Audio Card.
So my opinion is, if you want a modular supply... get one, they work great. A little bit of advice though, get more wattage than you will need, and get one of with an exceptional quality of build.