A new CPU, will require a new Mainboard and new RAM.

I don't think you're going to find much of anything that fits in your budget.

Shop around for a LGA1155 socket mainboard, a low end Intel i5, and 4 Gig of DDR3 RAM.

That won't be a "true" server class system, but then again, neither is your current setup.

When you're on such a shoe string budget, the only "factors" to consider is what you can actually buy for that amount of money.