The little Shuttle cases use to have 185W PSUs... the brand new Shuttle SN41G (with an Nforce2 motherboard) runs a 200W PSU. So I think the 200W would be fine--if it is of GOOD quality. I saw a SN41G with a XP2600+, 512MB of PC2700, a ATI Radeon 9700pro, a WD800JB, a CDRW/DVD combo drive, and a Turtle Beach SC running just fine yesterday. (he was actually playing some FPS game)
Note that power consumption is HIGHLY related to peripherals... if you're running 4 HDDs, I might say that 200W wasn't enough... but if you're running a single hard drive (or maybe even two), I think you'll be fine. Note that graphics cards are huge power hogs these days... but you can only have one... and I've seen the "worst" running in one of the SFF machines.
Best of luck!