It could have an athlon xp which while not the fastest thing in the world, is still usable. I'm still running two computers with athlon xp processors. Also, its a custom built computer so it may have some really good parts in it (we don't really know for sure). DDR memory is way over priced as it is out dated, but I have seen windows xp choke pretty bad with only 512MB of memory. More memory won't make it a quad core processor, but it could make it faster than it is. Alternatively, if the rest of the parts in the computer are good enough, he could go with a budget (or maybe not so budget) amd MB/cpu/mem combo for 150$ or more. Its a lot cheaper than a new computer.
Again, it kind of matters what parts are in it. download cpuz and see what the processor is. If its an athlon or duron, I would dump it. If its a reasonably fast athlon xp, I might consider 35$ for a 1GB stick of memory. Be aware, a 1 gig stick of ddr memory cost about 35$ where a 1gig stick of ddr2 (current technology) cost about 10$. You are paying a huge premium for outdated technology. I wouldn't spend a lot of money on ddr memory. For not much more than the cost of 4 gig of ddr (not that you board supports that much), you could get a new mother board, cpu, and 4 gig of ddr2 memory.
Also be aware, you might be able to run a new 1 gig stick and the old 512 meg stick together to get 1.5 gig (most of the time it works fine but sometimes it doesn't, and the only way to know is to try it). If you run a lot of stuff on your computer, the extra memory would help more than running the memory faster because they are matched sticks). If you are running a minimum system, with not a lot of programs installed, and not a lot of things running at the same time, memory might not make as much of a difference. if you have a lot installed, have an old windows xp install (they tend to accumulate unseen crap that eats memory), and or if you multi task (runs several programs at the same time), more memory might help.
By the way, technically speaking, more memory will not make your computer faster (it wont change the speed at all). What it might do is keep your computer from slowing down if you run out of physical memory (and it can really slow down bad with only 512MB of memory). If your computer is running low on memory, windows uses more virtual memory (it uses hard drive space as fake memory). As hard drives are worlds slower than real memory, it can really bog things down if it is using a lot of virtual memory.
Hit ctrl, alt, del, go to the performance tab, and see how much physical memory you have available at any given time? Not much? Then more memory will help (not speed it up, but keep it from slowing down).
Download cpuz (just google it). The first tab will tell you for sure what your cpu is. The 4th and 5th tab will tell you for sure what speed and amount of memory you already have.
Try to get a full parts list on your computer to see if it is worth salvaging, and or putting a little/lot of money into.
You want the folowing specs
os (we know its windows xp, is it a retail copy that can be used on a new motherboard?)
motherboard (we know its an ASRock - model K7s8x)
cpu (use cpuz)
memory (use cpuz)
go to start, control pannel, system, device manager, see whats listed under disk drives for you hard drive (it will probably only be a model number, but post it). See whats listed under dvd/cd-rom drives for your optical drive. See whats listed under dislpay adapters to see what your video card is (that board doesn't have onboard video so you have to have a video card).
Take the side panel off (you don't have to go messing with stuff, just look). There will be a metal box, maybe, 5x3.5 inches (the power supply), most likely in the top rear corner. What brand and model does the label say it is? At the bottom rear, there will be cards coming off the motherboard. You will see one goes to the back of the computer where your monitor plugs in (your video card). How many cards are there (you never know what kind of nifty toys might be there, if a friend built it for you).
Do you know what the case that it is built in is?