What should I do first ?

I'd go with the RAM, even tho stock anything is a freakin gamble. At least you can enjoy some games until you buy a better PSU.

I have no idea what PSU is so yeah, buy RAM.

RAM. Adding more RAM will make little difference to your power usage, but make alot of games more playable. A better PSU will only really help if you're adding a graphics card with high power usage, or if you already have a high powered GPU and the PSU can't keep up.

That's the order I'd do it anyway ;)

RAM, what would a psu gain you atm?

Does it smell when your computer is under load? if not, then go for RAM first.

Ram would be a good choice because to play today's games 4 GB of RAM is not a lot.

If your PSU is working without problems, why to buy a new one? Also RAM arent this expensive. Buy both at the same time or save your money for something else.

What everyone said, go for some RAM as it won't affect your power usage much. However, if you're planning on upgrading your PC soon or you feel that your power supply is worrisome, better be safe than sorry and get a better one. But by the sound of it, you have been using your computer as it is for a while and nothing happened yet, so I don't think it's anything to worry about.

I blew up a power supply once when I bought my new GPU (well, almost blew up). The specs were all wrong, I since bought a EVGA and it is extremely efficient. Point is, don't upgrade anything major like the GPU without first upgrading your PSU. RAM on the other hand won't cause any major issue.

in my experience, the came-with-the-case power supplies are fine until they die, which usually comes sooner than it would on a power supply bought separately. at some point you'll need to replace it, but for now you'll see more benefit from adding ram. of course if it dies a week after you bought ram and you don't have enough money to replace the power supply that would really suck.

if psu can handle the built so far go with ram.

Voted for RAM. Even though stock PSU is shit, it will probably last you some time until you can afford a new one. My "budget gaming PCs" (before current one, which actually has a nice PSU) never had a fancy PSU, and they worked really well, as long as the wattage is enough and some more just in case.

I'm going to be the guy who says new PSU should be the first concern.
Buy a good unit (SeaSonic only). The games can always wait, but if you're strapped for cash you can't afford to replace your
entire rig if the PSU blows and takes your components with it.

What CPU and GPU (Graphics Card) do you have?

bump

ram = more comfortable
good psu = safety

unless than generic psu is a "good one" (if that even exists?)

I'm going to say RAM, but don't skimp on a PSU, either.

The PSUs that come stock with most cases are utter trash and should be replaced. The PSU is the backbone of your system, and when it fails, it can take a lot with it. Once you get your RAM upgrade, I'd be sure to upgrade the PSU before anything else (at least to a quality PSU).

The most likely thing (by a wide margin) that happens when your PSU dies is that it, well, dies, without frying your motherboard. Unless you're pushing the limits of your PSU already, or it's been having issues, go with the RAM.
Also, when you decide to buy yourself a new PSU, don't buy a monster PSU. Buy what you need, not what the internet tells you that you need (luckily this whole monster PSU craze has been dying down).

Bump

I say go with PSU, without a reliable PSU there won't be anything left (even though chances are low but why take a risk). If you don't trust stock PSU that is. I also had monetary concerns so I went with PSU first. Nowadays you don't have to go for high capacity power supply like back in those days where graphic cards were power hungry. I had to buy 850W power supply.
Nowadays even GTX 1080 can be powered by 600-650W power supply. In the future there will be even more improvement in power consumption.

View attached image.

Go for more RAM.

A stock PSU is not the devil some may make it out to be. You will need to most likely upgrade it if you install some power hungry items (water cooler, new GPU [sli/crossfire], a power hungry cpu, or maybe you want to power a dozen hard drives, I don't know. If you have not gotten any power failures or errors by now, then you are good to go. Another stick or two or RAM will barely increase the power consumption at all.

When you do buy some RAM, make sure to match it up (speeds and timings) with what you already have to get dual/triple channel speed.

In the end, you will want a good PSU as well but unless your PC is randomly shutting down or having trouble POST then you can make do with what you have while you save up for that later.