The issue with rig-building is that economies of scale is VERY, VERY important.
No matter what you do, you will have to pay $400 USD (or so) to get your basic components - PSU, Motherboard, RAM, CPU, HDD, and so on.
After that, it's all about GPUs and how many your Motherboard and PSU can handle. So for $700, you may be able to get 1 or at most 2 GPUs in a rig with a cheaper PSU.
Comparatively, for about $1,400, you can get a much larger rig with a lot more hashing power.
The 7950s are the best bang for the buck, period. I've run countless payback tests on every card on the market for LTC, and nothing comes close to the 7950 unless you can get free power and buy 5850s ad nauseum (they are about $90 USD and hash at around 300-350KH/s).
Right now, I have about $1,900 invested in my rig (5x7950), and I'm expecting about 2,750 KH/s on the rig, or over 1.5 KHs per dollar spent. On a $700 rig, that number will get closer to 1.0.
Im fine with lower hash rates for now, I can upgrade in the future but I dont have the money for a better rig, and wouldn't for awhile. I was planning on using the revenue generated from mining to upgrade. Also, I dont pay electricity