The i5 750 is $200 pretty much dead on, but it requires LGA1156.
You didn't state if you had to buy the new infrastructure or not.
As for LGA775, you could get a Core 2 Quad in that range, not sure which one, as I never tell people to buy them any more.
The I5 is a fine chip, so long as you get the quad-core version. You can get a quad core version for $200 and a board for $100...but you would probably need ram as well (i5 uses DDR3; most 775 board use DDR2). In the end, it will cost you about $400.
The old Q8400 is a quad core 2.66GHZ chip (the same clock speed as the Quad core i5). It does not do quite as much per ghz as a i5...but it is close, and it is only about $160...a savings of $240 if you already have the board and memory.
Just make sure your board supports it...some of the earliest 775 boards never got updates to support the later processors. Also make sure your board will support a PCI-E x16 video card; if you have to change the board for the video card, you might as well go to an I5.
