Edit: Forget my prior upgrade recommendations. That board has no solid caps on it. I wouldn't want to try to rely on it for another five years. Time to retire that rig. (But only if you can afford to.)
If you buy OEM Windows 7 Pro 64-bit SP1 (the only one quasi-legitimately available from legit channels), then it will be "bound" to the motherboard, once you activate it with the product key. (You do have the option of installing it, and running it indefinitely, without a key, and still getting updates. Don't ask me how I know, though. No illegitimate cracks needed.)
But considering how long that you've probably had your system, you might consider doing a complete upgrade.
Another thing to consider, is buying a REFURB quad-core Sandy / Ivy Bridge PC, with 8GB of RAM, and adding an SSD and (optionally) a video card. You can get those, if you really look hard, for under $200, with the OS included. (Remember, if you were to buy the OEM OS through legit channels for your existing PC, you would be spending $120-140.) Also consider Haswell i3 refurbs too.
Maybe someone could like some cheap quad-core (2nd/3rd-gen Core) refurbs?
Edit: And if you only consider building new, here's what my G4560 browser build breaks down to:
Intel G4560 CPU ($60, Antonline on ebay, when it stock - currently, these are hard to find at list price online.)
Biostar B150S1 D4 micro-ATX mobo ($27 shipped, Newegg, they had a 50% promo code for a sale, and it was already marked down to $49.99. + ship)
8GB (2x4GB) DDR4-2133/2400 GeiL desktop RAM ($35, Newegg on ebay, bough these before the great RAM price increases took hold.)
Rosewill FBM-05 case ($25, Newegg or Newegg on ebay, on sale, these are like their other FBM cases internally, but they have a single USB3.0 port on the front, in addition to the two USB2.0 ports.)
Rosewill Stallion 400W PSU ($37 shipped, Newegg on ebay. This was in deference to the ThermalTake TR2-430 from BestBuy, often goes on sale for $40 + tax, or the EVGA 600W, often goes on sale at BestBuy's ebay site for $30 + tax, which wasn't available at the time.) Has a PCI-E 6-pin connector.
Adata SU800 Ultimate 128GB SATA 2.5" SSD ($40, Newegg on ebay, on sale, normally goes for $40-60. 3D TLC NAND, middle-of-the-road, but this was a great price these days, since SSDs have been increasing in price.)
Total: $224 in hardware, before the Windows license cost. Which can range from $20 from a dealer on CDKeys.com or Bonanza.com (sketchy), or $80 or $100 from Newegg (depends on whether it's on sale that week).
So, assuming that you could replicate my deals, and got Windows 10 from Newegg on sale for $80, that would be $304 for the PC. Which is really not too shabby.