Iv'e had a 3.5" HDD in my A1200 since 1993, no problems at all (+it's faster, cheaper, with more storage than a 2.5")I also had a 030/50/FPU/8mb all running off my standard PSU. So you should be ok. One thing you have to do (or at least I had to) was trim the metal shielding near the drive (2.5" to 3.5" converter came with instructions for this, as I got it from an Amiga dealer, should be instructions on the web somewhere).
