In answer to your question, probably more than you spent on it originally.
As the poster above said, you can reuse your peripherals, and you can also reuse your HDD (make sure you clean it out thoroughly for any nasties before taking it across to the new system.) That said, unless you give us a better idea of the specifics of your HDD, it's hard to say precisely how good it is. The case could be alright, but you have to consider that modern gaming hardware generates a lot of heat, which older/non gaming cases aren't necessarily built to deal with, so you may need a new case.
You'd need a new CPU, new Motherboard, new RAM, new GPU, new cooling solutions and a new PSU for sure. You may need a new HDD, a new case and a new OS.
Gaming PCs are big money. You get what you pay for, but it still can be daunting to someone new to PC gaming, especially if you've come from consoles which are a lot cheaper. To get a high end rig, you need to pay out about £900+ minimum, and to get the best with all the bells and whistles, you'll need in excess of £2-3k.
However, don't be put off, as PC gaming is amazing (hell, otherwise we wouldn't all be here.)
However, whatever you do, you've got people who you can turn to for help and support on these forums, so definitely don't feel alone.
Good luck with whatever you choose,
-Nih