I thought anyone with any sense did this?
Anyhow, as others have said, a good psu is key, something from Enermax or Corsair costing about £100 should see you right.
Make sure you either use a case with great airflow, or you can retrofit the case you have with 120mm fans, sort the cabling out to stop pockets of hot air forming and strip out any unnecessary components.
Something many people over looks is to remove the graphics card, especially ones with their own fans, and just use the on-board chipset as this reduces power being taken up, avoids a system crash due to the graphics card being stressed and of course reduces the system heat a fair bit too.
You can probably forget about transferring the files via wireless's - the uneven speed, plus the high chance of the hd in your leeching deck being fragmented = a recipe for cyclic redundancy errors abound. So unless you want to stream movies, best to defrag and simply burn the files.
If you have some spare money you could also pay an electrician to make a "clean" power source for the pc, or a cheap option would be to get a decent uninterupted power supply to reduce your deck resetting in the event of power spikes, storm interference or other electrical issues.
Get a decent hd if you can, Seagate are great and give you a 5 year warranty, some firms such as Western Digital only give 3 years and will send you a second hand "reconditioned" drive as a replacement. I've never had a Seagate drive fail, unlike pretty much drives from all the other major manufacturers. Having a 16meg buffer and 7200 rpm helps..
EDIT: Oh and I use Emule, much better than the leeching hell that is bittorrent