what is this 250W power supply currently powering? it should be fine under normal conditions it's just the startup load I'm concerned about. Hard disks only draw large(ish) currents when they spin up.
Low power graphics and a non P4 cpu and you shouldn't have a problem. I'm pretty sure you'll be fine if you've not had any bad experiences with it so far.
[edit] just checked your "mobo" link, in fact it's a shuttle barebones system to save others the hassle of looking. Assuming you're running off the integrated graphics then you'll have no problems power wise. (The only problem I've had with these systems in the past is them suffocating with fluff inside, occasional hoover/clean-out solves this.)
AMD AthlonX2 5000 black edition @2.8ghz, 2gb pc5400, AMD/ATi hd3850, creative xfi music, 24" hp widescreen 1920x1200, ECS KA3 MVP mobo