an i5 uses built-in GPU on the CPU. Which is pretty much the same concept as having an IGP on the motherboard.
The new Sandy Bridge i5 has the best graphic performance, which eliminates the need of a discrete graphic card. But the price of the CPU and motherboard alone can cost around $300.
The best option, price wise, would be a dual core i3 (also has on-die graphic, yet lower performance) with LGA1156 motherboard. The other alternative would be an AMD platform with at least a dual core Athlon II X2 and an 880G (HD4250 IGP) chipset motherboard. You may not need a triple core for HTPC.