You should take a look at this thread on nano-reef.com and it will answer most of his questions about how to make the changes you need to make to a halogen for it to work.
http://www.nano-reef.com/forums/inde...c=91165&hl=70w
Then buy a 70W bulb from this guy on e-bay (I've got 5 bulbs from them now and am very very impressed)
http://cgi.ebay.ca/70W-70-watt-14K-H...QQcmdZViewItem
This should work and be one of the cheapest ways you can do it in my opinion.