Games are not multithreaded, so using a heavily-multithreaded CPU will hurt performance rather than increase it; which is why Intel does better with gaming and single threaded tasks.
The Athlon X2 is designed specifically to be a cheap alternative to the Phenom 2, which actually is old. These Haswell Pentiums can run circles around older AMD processors. There are still modern games that only run on a single core. I was looking at Assassin's Creed 4 just last night using only 1 core, no HT. Pretty pathetic.
What is your definition of "old"? A HDD is a HDD. Long as it isn't IDE or a first gen SATA clunker you'll get basically the same performance. The best drive for a main drive would be an SSD hands down but your budget doesn't really permit this. I say utilize your budget fully to put into your system if you already have drives. If you have a larger drive like a 750GB or 1TB you aren't really using or can transfer the data off of it you could short stroke it to get a "smidget" more of performance but nothing close to what an SSD can offer.