Upgrading store-bought HP 700-339

Are you sure it is a K sku CPU? Most of the spec sheets online refer to it as a basic 4790. Given your board only has two RAM slots, you'd need to get a fresh new kit of 2x8GB RAM, unless the slots are only filled with one DIMM. Not to mention, even if there is an open slot, you'd need to find another identical stick if you wanted to upgrade it, otherwise you would run into some compatibility/timing issues.

1. It really depends on why you need the extra RAM, are you editing/rendering, gaming, or? If you're only gaming, then I'd stick with 8 GB. If you're rendering, then that case is pretty terrible for thermals, so you'd want to invest in a higher quality case as well.

2. Do you feel a NEED to upgrade the board? What would you be getting out of it, aside from possibly more RAM slots? The issue with prebuilts is that they're usually not built with upgradability in mind, so they usually only build the system around ONE set of components. That being said, it's highly unlikely the case would work with a standard ATX board.

3. I'd recommend getting a bigger case regardless, prebuilt cases are generally terrible for future upgrades anyways.

4. Yes. YES. YES. An SSD upgrade should be FIRST on your list. Just be sure you have your windows product key ready, along with all the other necessary information to move windows onto your SSD.

The 460W might be an issue, given they don't specify what kind of outputs it has; for all we know, it might not even have the correct power outputs for the GPU you want to upgrade to.

If I were you, I'd upgrade in this order:
SSD, Case, PSU, New Board(Preferably an H87/97), GPU, then RAM.

For the SSD, any 240 GB variant will work. I would recommend these parts. https://pcpartpicker.com/list/q4WnpG I can't find any sort of reasonably priced H97/Z97 boards via pcpartpicker, so you may want to look into local stores/ebay to solve that issue, just be sure to be careful about the condition of the board. If it had previously bent pins, DONT BUY IT. It's up to you whether you want to try and run the two different sticks together, though I wouldn't advise you to do so.