First off, 2gh/s wont be happening with 700 dollars, but you can get pretty close.
This list is made assuming new GPUs because 5xxx series are very hard to obtain. You will find just about no store holding them in stock at reasonable prices.
Useless parts:
Sell the 3 working athlon 3300's at $10 each for $30, sell the random GPUs for $40 total (8800 for 20 bucks), gain $70. Optionally sell the core 2 quad and sempron but I wont cover this in the calculation, however those will net you about 100 bucks or more
1 single 16x slot mobo on 775 chipset. No Psu with board. DDR 2 ram. 1 sata connection.
Add the spare 350W PSU and P4 processor, one of the 512MB DDR2 ram sticks, run it on the 160gb drive. Add Radeon 6950 @ $239, reflash via hardmod to gain about 400-430mhash/s, connect via supplied Molex -> PCIe, PSU will handle it if you keep CPU usage down to a minimum
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150523You are down to 531 dollars. You wont have enough to buy two more 6950's and build a system around them, even that would only come down to 1.2ghash/s
Best bet is buying used old computers with PCI-e slots (even x1 will do fine) on sites like craigslist or ebay at about $30-$50 a piece. Then fit them with used 5830's 5850's, 5870's.
You might just be able to stretch to about 1.6ghash/s if you find 4 5870's at $150 each and buy a used PC for each one at 25 dollars. That's a bit of a stretch though.
If you want to aim for a lower, more realistic goal post here
