DDR5 isn't a thing.
GDDR5 is a thing. It's graphics card memory.
Zoostorm actually does pretty good deals for PC's, a friend of mine found a really good one for about £300
I can see why you would want to upgrade this one though, it's pretty low end stuff
Just as advice, RAM is expensive upgrade to 8GB instead for now. A 750w PSU is overkill as well, you would be better off spending money on a high efficiency 500w or 600w PSU (look at Corsair)
Also that graphics card is horrendous. You would be better off waiting a little bit and going for something like a GTX 750 or something. Price to performance is much higher.
The description for that Zoostorm PC isn't very specific by the way, could you find out what motherboard you have maybe?