I'm kind of late to this thread, but I would look at your present motherboard and see what is the best CPU that it can take. If it's going to be outdated soon, you may as well build a new PC now rather than have to do it a year from now.
Think about what you want to do with the PC, and how long you want to have. I always say never go cheap on your PC, and always make sure you give yourself room to grow. Spending a little more up front may save you money down the line because you won't have to upgrade as quickly.
I built my PC in Feb 2011. I7 2600K processor, 12 Gigs of ram, GTX 580 GPU. Almost four years later I've made one upgrade, a solid state drive which was more of a luxury purchase than a necessity. The GPU is getting a little arthritic (still totally capable though), but I can upgrade it without having to touch anything else or worry about a bottleneck.
For the power supply 500-600W 42 amps all on one 12V rail (very important, not the size but how the power is delivered). I have a 650W corsair I brought over from my last PC before this one, probably 8 or 9 years old. I'll swear by Corsair products, my PSU, Ram, SSD, all Corsair, never an issue.
Make sure you CPU and GPU have similar ceilings. No use having a 1000 ft ceiling in the video card if your processor only has a 500 ft ceiling.
Get a bunch of ram, it's cheap, 12 or 16 gigs, won't have to upgrade it for A LONG TIME.
I'd recommend just building a new PC rather than upgrading. Odds are your current system is getting outdated and you'll end up having to build a new one soon anyway. Because once the motherboard and the processor are maxed out you're done. Once you change the motherboard you're reinstalling the OS, all the programs so you may as well have just done it now.
Plus if you keep using your current PC you can look for sales on each component and build more cheaply because you don't have to have it done right now.
Sorry to be so long winded, I just see a lot of people make the mistake not realizing spending less money in the beginning may cause you to have to upgrade earlier and more often.
Just my 2 cents