If you have a bit of time, sign up for newsletters from TigerDirect, Newegg, Microcenter, and anyone else you can think of. Also, create a wishlist on Amazon with all of the parts you want in it. Amazon's prices fluctuate day to day and sometimes throughout the day. I was on a tight budget as well, and keeping a close eye on sales really helped maximize my dollar. I started looking about 3 months before I built. I also ravaged my house/garage and sold things on Craigslist to fund my CPU project. Craigslist sales alone netted me about $300, which is cash I used to buy things when they went on sale. I made enough selling stuff that I was able to get a some nicer components than I had initially planned.
Microcenter doesn't seem to have the rock bottom sales that others have, but they CPU/Motherboard combos can't be beat.
Some of the deals I was able to get:
Seagate Green 1.5 tb hard drive from TigerDirect for $50
Windows 7 home premium 64 bit from Newegg for $79
Thermaltake 430 W Power supply from Newegg for $20
Intel 330 Series 60 gb SSD from Newegg for $59 (came with a free copy of Civilization V too)
I must admit that I did end up selling the power supply and getting a different one because it didn't fit in my case, but I still got the deal.