Will an i5 2400 bottleneck a GTX 980?

the only thing that may bottleneck your cpu would be the lack of PCI-E 3.0. The sandy bridge 2XXX series processors only support PCI-E 2.0. the cpu itself wouldn't be bottlenecked, but the pci bandwidth maximum would be. putting a pci-e 3.0 card into a 2.0 slot, it will be as if the card is running on 8 lanes of pci-e 3.0 instead of 16. i don't THINK a gtx 980 will fully saturate a pci-e 2.0 slot, but it may come pretty close in some benchmarks, games, instances. this will become more obvious and actually cause bottlenecking if you go to dual slot sli in the future, as the 2.0 lanes on your motherboard would operate at x8,x8 mode, which would be pci-e 3.0 at x4, x4, which is very bad for almost any high end graphics setup.

i would recommend a new build for better features, pci-e bandwidth, usb 3.0, native sata 6gb\s controller, and generally better future-proofing. If you purchased a sandybridge on the 67 or 68 series chipsets, the next gen, ivybridge was just a die shrink with a new chipset (77 series) and maintaining the socket type (LGA 1155). A die shrink just means more efficient with less power, but it has the same architecture as the previous generation, usually comes with a feature upgrade or two (sandy to ivy gave us native sata 3, pci-e 3.0) and doesn't have sufficiently high performance gains. . tics and tocks. haswell is the latest intel chip with the 87 and 97 series chipsets and is a completely new architecture. its die shrink, broadwell, should be coming out in the next few months. Tics and tocs... tic is new architecture, tocs are just die shrinks.

basically, if you bought a pc on a tic, you should stay on the tic, as new architectures mean massive feature and performance increases compared to the previous architecture. waiting for the toc, or the broadwell processors in the current state, wont yield exceptionally advanced performance or features over the current offerings available. the only new feature rumored is to be DDR4 motherboards, but the price of DDR4 memory is very very high right now. now is a great time to buy a new pc as the prices on SSDs are now very affordable, memory speeds are much faster for much cheaper, and graphics cards are absolutely stunning to play on with a fully featured motherboard and processor. if you have time to save up, i would highly suggest it, then drop your new gtx 980 into that sweet freshly minted pc.