Yup, I place a bit smaller than a pea-sized drop in the center of the CPU and it works great. I love that paste, especially when you are putting it on new waterblocks for GPUs, easy spread and isn't watery like some pastes.
At $500, I would be tempted to put it on one of the altcoins like Terracoin or PPC but I would still look at something with a much higher hash rate if you were going to drop $1k on something. I just grabbed ~16-17 GH/s of ASICs for $200 so I can't really say too much
Is this with ribbon cables or without? With PCI-e x1 powered cables you can run on pretty much anything. Other than doing something like that, the only LGA1155 board I found with 4 full x16 slots that will work with 4 GPUs is this: ASUS P8Z77 WS but it is out of your price range.