Is it better to upgrade a slightly old computer or just get a new one?

That depends on how old is old, what is in it {specs}, and what you desire from a computer.

And whether you want to be able to upgrade in the future..(ie PCI-Xpress, DDRII RAM, SLI/Crossfire etc) and what you want to use it for (internet, gaming (3D/educational/strategy/online), word processing etc)....

Well i dont know how old my computer is. I want it for gaming and its a Gateway. It does have a pentuim 4 procceser though.

The pentium 4 has been around since 2002, and I'm guessing that you have a Dell. If your computer is that old, and it's a Dell, replacing parts could get expensive. Take their power supply for instance, they have them made for their computers, so you can't go out and buy and install another retail PSU.

Even if it isn't a Dell, if it's four years old, I wouldn't invest any money on upgrading, I would either build a new machine, or purchase a new or newer machine.

We need the model number to see its capabilities.

How old is it?

Does it have Hyperthreading capability?

What kind of video adapter do you have? Does it have its own RAM or does it use shared system ram?

If it is a seperate video card, is it AGP or PCI and is it 4x or 8x?

If gaming is your priority and it's more than just a couple of years old, you might be better off building/buying a new one especially if it doesn't have a screaming video card/adapter now. You have to consider things like how big a power supply you have (which was probably marginal if the computer was commercially built like a Dell Or HP or whatever, how much RAM and what kind (and if it isn't DDR or DDR2), and what the capabilities of the motherboard are.

Computers age very rapidly as technology advances geometrically, and anything other than adding more RAM or a better video card (if your power supply will support it) becomes a not very worthwhile investment after just a couple of years and maybe less, depending on what you had to start with.

By the time you replace all the proprietary part on the Gateway you will have spent the money that could have purchased haft a decent gaming computer.
IMO you should make this computer a secondary (bacup) computer. Spend a little be more than the cost of upgrading your present computer and buld yourself a gaming box. That gives you two for one. Use this computer when youcannot get online with the gaming box.

IMHO

From what I've seen of Gateways, and heard about upgrading them, it seems remarkably hard. If yours is one of the ones with a tiny case, there's not gonna be much room for upgrading (no AGP slot, 2 RAM slots total...). I'd just do like Acklan said and get or build yourself a decent gaming computer.

BTW Those old computers make great TeamSpeak servers and other servers. If you're running a P4 if you have at least 512 mb ram or around 1 gig (wich I recommend) then you can run all sort os like BF2 servers and stuff if you have a half ass connection.

I use my old PII 233 with 128mb of ram and I run a team speak server nice and smooth. We were 6 on it the last time and we had no lag issues and we were using the highest quality codecs.

Personally, I would go for a new system all together. You could sell the computer you're using right now and put that towards a gaming rig. Trust me you don't have to spend a whole lot these days to get a great computer I spent $500 on my computer and I have no problems with the latest games and the future doesn't seem to pose much threat to my system either.

Plus, with the new system it would be much easier to upgrade than the Gateway in a few years.

I agree on the first point but not the second. You can hardly get anything for used computers right now. If possible you should have a second computer. If you own it it does not cost, and thnk of the members that have to run to a frined or the library to use the computer becuse of malware.
That is just my take.