Just a suggestion, this happened to my desktop Shuttle system when I fitted a Radeon 9800AIW. Essentially, the PSU wasn't supplying enough power on the 5V rail which meant then when everything tried to draw on it at boot, the whole thing just kinda... well.... stopped. Just like ur describing. Occasionally it would manage a boot so it had me confused for a fair while. Stuck a bigger PSU in and it was all fine after that. Could be something to look at anyway. Or it could just be somekinda short.
Sorry I can't help more.
*edit* Should mention this.... the temporary work around as suggested by ATI when I had this problem was to leave the molex on the Radeon unplugged until the post beep then plug it in - cos there wasn't the almighty inital draw by everything at the same time it actually worked. Was a bit scary tho lmao btw I am NOT taking responsibility if u try something like this!!!