Desktop Rebuild

A little background:
I built my gaming rig (in my signature) about four years ago. I am a lot less up-to-date on parts these days, so I came here to get an idea of how to rebuild my computer.
My motherboard will not post anymore, so I'll definitely be getting a new Mobo, CPU, and RAM. Graphics aren't as important to me anymore, but I would still rather have a dedicated GPU rather than onboard. I'd probably use my HD3870 (PCI-e 2.0 x16) until I save a little more money to get a new 3.0 GPU (as long as this old card will work on a newer Mobo).
I plan on keeping my Antec 900, 320GB HD, and optical drives, and wouldn't mind keeping the 520W PSU if it has enough power to run the new motherboard/CPU.

I've been looking into Intel CPUs a little, and I'm thinking I'd want to go with an i5. I know that RAM is even cheaper than it was 4 years ago, so that isn't an issue for me. Mainly I am looking for suggestions on a good motherboard, a compatible CPU/RAM, and suggestions on if I need a new PSU or not.

The computer will be used for light gaming (CS:GO & older, maybe some new RPGs), programming, and HD video. My budget is around $400, but that is really only for the Mobo, CPU, and RAM (excluding GPU and PSU).

Let me know what you guys think. Thanks for helping an old CoFo member out in these dark times

Have you tried resetting the cmos on the motherboard?

I gave it to a computer repair shop to get a free estimate on the rig a few weeks ago. He ran some diagnostics on it back in his shop.
I don't have a lot of knowledge on motherboards, but he said it wouldn't post and I would need a new one.
I think he understood that I built it myself, and wasn't about to chip out $120 for the repairs + whatever the new parts cost, so I don't think he would just say this to try to sell me his services.
How would I go about resetting the CMOS?

You would either remove the CMOS battery whilst the computer is powered off and then reinstall it several seconds later, or use the clear CMOS jumper on your motherboard and move the jumper across a pin and then move it back after a few seconds whilst the computer is powered off.

I am currently at work, as it is my only access to a computer.
I'll try resetting CMOS this afternoon and update you guys then.
Thanks!

I reset the CMOS by removing the battery, waiting 5 minutes, and trying again with the battery in place, still no beeps/post.
Any more troubleshooting help or suggestions on parts for my rebuild?