I fully agree XavierMace above on every point he makes, your system is a 2core duo, so your running DDR2 ram with a limit of 8gb and while you can get quad cores for a socket 775 (I believe) motherboard they've been out of production for 3+ years now and much like getting DDR2 ram its going to cost you more than any higher spec upgrades that are still in production, 4gb of DDR2 ram costs more than double that of 4gb of DD3 currently, while a quad core processor for such a system will cost anything upwards of £100 to silly money. Its currently cheaper to build a system from scratch (you can keep hard disks, though u'll need a repair for your OS) than it is to bring your 4gb duo to a 8gb 2quad, I know this for a fact as I scratch built a new system at the start of the year from a slightly higher spec system than yours is currently.

In regards to over clocking and altering bios functions let me second the voice not too touch these, I scratch built my system and I have a unlocked processor and motherboard with overclocking software but I won't touch it for too reasons, one I simply don't know enough about what I'm doing (there's a ton of crap too consider, god I get scared playing with my fan settings too much!) and I'm far from computer illiterate. Second Overclocking reduces the life expectancy of your components because your running them beyond the manufacturers guidelines (which even with components designed for over clocking voids the warranty instantly) which shortens the life expectancy from those components which for me personally is something I cannot afford to do as i'm on a low income as my partner has health issues which prevent me from taking on any job on a full time basis. However despite my income issues a system change rather than upgrades is the best economical option by miles and if you can't afford to do it now then put the money aside that you were using for upgrades and make do until you can overhaul because otherwise your literally throwing money away.

Finally those who are looking are GPU upgrades (That's next on my list also because I want to play Star Citizen) wait until July because in June AMD are releasing there R9300 series cards which means shortly afterwards the R9200 series will take a massive price cut so you'll I'd guess be able to get a 3-4gb R9290 for around £140 if you shop around which is a amazing card currently right towards the upper end of the market. I feel your pain, I've been in your exact position just a few months ago and I know your machine was able to run any game released in 2014 without hiccups but in 2015 games are coming out on the new Unreal engine and the new Unity engine that just wont run on your machine and as the year goes on this situations just going to get worse and worse as games developers are able to utilise more aspects of multi core processors with high powered multi core GPU's.

When I run gigantic or immense my physical memory use is about 8-9 gB.   Of course that doesn't directly translate because a computer and OS use resources differently when they have diffferent stuff available, but still.   With 4 gB and a 12 gB page file you have enough memory to run the same, but the performance is not going to be good with the HDD constantly thrashing.    One critical thing about GC3 is that it exploits hyperthreading --- for example, I have 2 cores and 4 threads and they are all running at 70-90% usage when the game is running.  (Easy to see using the task manager in windows)   That's good efficiency, and what the CPUs were meant to do, but it puts a real strain on the system if a HDD needs to be accessed to support it. 

I went from 8 gB to 16 gB on my 4 year old system and it made a world of difference.   I might add that with the last 3 or 4 patches I haven't seen a UI freeze at all.  So in summary I would say things are getting a lot better but an old system with 4 gB memory is asking a lot if one expects smooth game play, and imagining running on one of the larger sizes is pretty optimistic. 

Well, maybe those guys are Stardock are geniuses, who knows.