To be honest, with a 300 watt power supply if you were to put a video card in there it would be very bad. I doubt the PSU even has an 8 pin connector for the video card. So, I second fb39ca4 on that. If you aren't quite ready to upgrade everything you could upgrade with a 700-800 watt PSU and a decent video card then in a couple years use the same PSU and replace everything else.
edit: the life of all parts is sort of a mixed bag, most have warranties of a few years (typically 2 or 3) so you expect them to last at least that long. If you treat it like a car, they have 3 year warranties for the most of the time and can be driven for 10 years or more! Things like overclocking will shorten the life of parts typically, but I doubt you are overclocking much. General use PCs can last a longgggg time!
your processor is quad core and a very good processor and you just have to buy a 600watt or 550watt psu and a good card like gtx 460 if low in budget then get ati 6790 and you will be able to play games all that will come in next 2 years then change you this pc to i7
If you're looking for a small upgrade without spending a lot of money (say, budget $100), you can buy just a better low profile video card which doesn't require auxiliary power (e.g. HD 6670)
Otherwise if you want a substantial upgrade, its likely you'll have problems with your tower fitting a nice GPU and PSU. To get this to work it will require a bit of ambition on your part:
Buy a new case $50-150, at least a mid-tower. That G520 tower is what Dell calls a mini-tower and many of the video cards worth the upgrade are 10-11" in length, so they won't fit, also possible problems getting a PSU to fit, and a CPU air cooler if you chose to do that (see below).
Buy a quality 500-600W power supply $70-100.
Buy an HD 6950 or GTX 560Ti, $220-280.
Buy a decent aftermarket air cooler for your CPU $30-50. A reasonable overclock to the Q6600 CPU on air cooling is 3.0Ghz, and the ceiling on air cooling is about 3.6Ghz. ( I have to give you the disclaimer that I dont know what your motherboard is capable of in terms of overclocking. )
If you don't want to do the overclocking, dont buy a CPU cooler and just buy a little cheaper video card like a HD 6870 or GTX 460 (note: these cards will still require PSU/Case upgrade) to limit the bottleneck, those are in the $150 price range.
Pull this off, and in the future you can simply spend $300-500 more on new CPU/MOBO/RAM and your system will be completely overhauled, but for now if you're on a budget, your CPU is worth keeping.
My intentions are to do nothing that requires too much intensity. The only game I would play for the next 5 years is BF3 (judging from my experience with BF2) and I don't mind having that on medium settings.
I also spend a lot of time with the whole adobe suite as I illustrate and animate for a living. Basically I would love for it to be as simple as "just buy a new gpu and that should carry you through for another 4 years at least", but I doubt that.
I want it to be reliable, and last a long time.
The fact that my MoBo is already 5 years old is what worries me.
I am on a budget, roughly £100 tops ($160). If I buy a new GPU (probably the HD5670 because of it's low power) and my current PSU can't handle it, would I have warning signs or will it explode suddenly? My brother is currently throwing out his old Dell (coincidentally another from the same generation) and his PSU is 375w, would it be recommended to just grab his PSU or is it bad news getting one second hand?
My worry with that though is. My brother bought his PC in 2007 and it worked fine, then he took it to his student accommodation the year after and it wouldn't boot up there, couldn't get past the BIOS screen, made strange beeps and flashed its lights. Didn't bother him much because he was a typical sociable student lol.
However, the year after that (2009) he moved into his own apartment, where the computer worked absolutely fine again (and continues to do so). I suspect the issue was with the outlet at his student accommodation, would this have damaged the PSU somewhat?
The good thing is he didn't use it for a year so it's only actually 3 years aged instead of 4, but could that dodgy outlet have caused permanent unseen damage to a PSU?
at least 720p, I really don't mind a general medium look. If it matches consoles, I'm happy.
Monitor reaches 1080p but I don't expect to play that high on my budget. As for other uses for a new GPU, I'd use it for Photoshop and Video editing mainly. (by video, I mean animation, which is generally far less intensive than full fledged film)
well i'm running a Q9550, marginally better than a Q6600, and am having no troubles at all, also the psu and then gpu can be happily reused in the next build, so its not a waste. Looking at the Bf3 review in toms, it doesn't scale with cpu, so virtuall any old cpu will help, but i've heard people going from dual to quad getting a better feel.
So that quad will be fine for a good long while.