With £/$XX what could I get to increase performance?

CBA to read? Important things are Bold.(You could literally just read the bold, and it would make sense)

Ok so here's my current(appalling) custom rig:

Case: Some terrible Acer Power Case that I've torn the front off, but don't worry, cooling is sufficient.
Motherboard: E946GZ (an OEM LGA 775 DDR2 board)
CPU: The highest my Motherboard supports, a Core 2 Duo E4700 2.6Ghz
RAM: 2GB DDR2 333.3mhz, there's another 2GB dimm in the post, I don't know if it works though, but I will have 4GB
Graphics Card: ASUS GT 440, this thing is the cheapest DDR5 card on the market but it does alright.
HDD: Some old OEM wester digital @ 200GB's(SATA) and as a boot, an IDE 80GB from hamaichia or w/e
PSU: 600w cheap as anything Alpine.

So yeah, it's terrible, but it's good considering I have no income and I get about a total of £100 a year from christmas and birthday combined (yes I'm not a self sufficient adult, I'm 16 but what can I do about it?)

Anyway, as of last week, every 4 weeks I get an income of £100 (for reasons I'm not willing to disclose), however about £55 will be towards a seasonal buspass and food for college.

So lets say monthly I get £40

How should I best spend this to improve my computer?(I'm not a no lifer, I will be spending some on going out but I get that money elsewhere)

I mean obviously I see shiny keyboards for £50 but I need good hardware that actually works, so what should I upgrade next and what should I get?

I was thinking getting an LGA 1155 board, get some nice 4/6GB DDR3 RAM and buying a new processor, however this would all cost about £100 and it wouldn't actually be a great improvement, I'd have to spend a lot more on a processor (about £150 for a nice i5) to even get something better than my current processor(according to cpubenchmark.net anyway)

So I'm convinced there is a better way to spent that (potentially £160+) money on something else, black ops and other new titles come out soon, I dont want to save up for 3+ months just so that I get a slight boost out of my computer in gaming, so yeah, I'm pretty stuck D:,

VVVVVVVVVVVVVVVV

what should I do? What should I spend my money on in order to get better frames in games and just a general better machine for gaming.

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Current frames in new games:

Black ops: 20FPS with huge random frame drops @ 1440x900 and low-ish settings
Arma 2: 30FPS on medium settings
GTA IV: about 20FPS on low settings
Skyrim: 30FPS on medium-low

So yeah, it's 'okay' but if I wanted to get something like battlefield or newer games, my computer would get in-playable frame rates with the game looking like crap on lowest settings.