Good idea, but my old computer gave up the ghost yesterday, which is why I'm in a hurry to get a new one built.
That's how I got to upgrading, late last year

. I know the feeling, but take at least another day or two to finalize, if you can.
If a gaming PC, this is how I'd use up your budget. You have $1500, so while there may be no need to spend it all, you really should do a forklift upgrade, with no old parts, save for transferring the HDD. Take the good parts from your current PC, a bit of other salvage, and make a secondary or donor PC with it. 8 years is quite good. Let it go,
entirely 
.
If you have an old monitor, get a new one, unless it was a high-end Dell, Apple, NEC, Eizo, etc.. Like an SSD, the benefit for the cost is good enough that you'll wonder how you managed this long, after you get used to them (unless you look at 2011-12 prices

).
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Core i5-4690K 3.5GHz Quad-Core Processor ($239.99 @ Newegg)
Motherboard: Asus SABERTOOTH Z97 MARK2 ATX LGA1150 Motherboard ($165.99 @ Newegg)
Memory: Mushkin Stealth 8GB (2 x 4GB) DDR3-1600 Memory ($65.99 @ Newegg)
Storage: Crucial MX100 256GB 2.5" Solid State Drive ($112.99 @ Amazon)
Video Card: MSI GeForce GTX 970 4GB Twin Frozr V Video Card ($355.91 @ Newegg)
Case: NZXT H230 (Black) ATX Mid Tower Case ($69.99 @ Newegg)
Power Supply: Corsair Builder 600W 80+ Bronze Certified ATX Power Supply ($34.99 @ Newegg)
Optical Drive: LG GH24NSB0 DVD/CD Writer ($17.99 @ Newegg)
Operating System: Microsoft Windows 8.1 (32/64-bit) ($122.98 @ Newegg)
Monitor: Dell P2714H 60Hz 27.0" Monitor ($309.99 @ Newegg)
Total: $1387.81
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2014-10-05 22:06 EDT-0400
Just get a start menu replacement and tweak settings for now, rather than buying Windows 7. If they don't screw up 10 between now and release, look for cheap upgrade offers when it comes out. It's looking good so far. I have to use 8.1 fairly regularly, am halfway decent with it, and
still dislike it, but it just doesn't make sense to get an OS already past mainstream support (OK, technically it has a few more months, but it may as well be over).
I wouldn't go with a cheaper mobo and CPU, at the moment, simply because the savings aren't much, unless you go to an i3 (with $1500 max, that doesn't make sense even for a non-gaming, non-productivity desktop) (with an i5-4460 and ASRock H97M Anniversary, I only got ~$35 less, on a $1000+ total).
SSD+HDD is pretty normal, now. You can just plug in your current 1TB after installing the new copy of Windows, and use it for a data drive, if you don't need more space. Also, if you might want more than 256GB, but still not a lot, a 512GB SSD would not be a bad buy. The way it works is totally different, but like HDDs, SSDs need some free space to stay really fast, so getting one and using up 90% of the space for static data is a bad idea.
If not gaming, Haswell's IGP is likely to be good enough (so, -$350), or a much cheaper video card than specified. The GTX 970 is the clear bang/buck king for gaming, ATM. If in your perusals, you find a GTX 760-770 performance level to be good (IE, want to play new games), and the cost is good, note that the their replacement, the 960, should be out within weeks (the 970 are OOS everywhere, FI, because they just came out).
600W PSU just because the cost is right, and it will be quieter due to the overhead. The 500W model is $5 less, and still plenty of power (both are overkill without a video card, which you should only need if gaming--at least a high-power one). With a GTX 970, you'd be looking at around 250-300W peak draw, and more like 100-125W without the video card.
Case choice is flexible, that just happens to be a nice case at a nice price right now. Cases have, on average, improved, too, and a new case can get you front/top USB 3.0, which is becoming common for storage. For quieter cases with door, the Fractal.Design Define R4, Antec P280, and Corsair 330R are good alternatives. With no quieting in mind, and/or no door, the Corsair 300R, Fractal.Design Arc Midi R2, NZXT Phantom 410 and 530 series, and Phanteks Enthoo Pro are all hard to go wrong with for the money, having drive rails, good build quality, cable management provisions built in, and good cooling.
I'm sure there's better bang/buck monitors. That's not my forte. I just know that's a good one for general use, with low enough input lag for gaming, as well.