First time building a computer. (under £300)

Hi ~

This is my first time posting here, I've been reading posts and articles here for some time now since i want to build my own computer now.

I need some help and advice just so I know I'm doing things right. So please have a read and it'll be great if you can help in any way possible

Thanks

Approximate Purchase Date: This week or early January
Budget Range: under £300 with shipping

System Usage from Most to Least Important: General school work (I do Electronic Engineering so programming and simulation stuff), I'm not looking to build an expensive gaming system but I do want to be able to play games on it like Far Cry 3, Crysis 3 etc. Also, I dont want to build something that won't be able to play games in about a year or something (or that asking for too much?).

I want it so that if need be I can upgrade it so it can run up to date stuff in about a year or two

I am not including the monitor in this. I might use my current one or get one separate one later.

I've always had my computers run for like more than 4 yrs.

Parts to Upgrade: Starting fresh.

Do you need to buy OS: No

Preferred Website(s) for Parts: UK websites, Amazon, anywhere good and not pricey that uses paypal

Location: London, UK

Parts Preferences: These are the components I have picked so far:

Motherboard: ASUS P8Z77-V LX - from amazon

Processor: Intel i5 3570k - I saw this on Cex. Is it ok to buy processors from cex, I dont know if theyre selling it new or what. Should I buy it from somwhere else?

Graphics card: Geforce GT 630 - I want to get a budget graphics card, I found this for £43. If there's a better one for gaming at a similar or lower price please tell me

I am also considering GT 440 or GT 430.

RAM: 8GB DDR3 1333Hz (2x 4GB) - Where can I get RAM for the lowest price? I usually buy from mr memory and I think they have this for £27 or something. Is that ok or can it be cheaper?

Case: I saw some cases on Scan.co.uk I'm not sure if I should get the case and PSU together or seperate. And honestly I'm not sure what power I need. I think I need 300W. Can someone help me with this?

Overclocking: Maybe

SLI or Crossfire: not really

Your Monitor Resolution: 1440x900 current monitor

Additional Comments: Nothing too fancy, keeping it budget freindly. no unnecassary 'bling'. I will just want to be able to do photoshop work, play games etc

And Most Importantly, Why Are You Upgrading: I have a dell optiplex gx620, it runs well but is so out of date, it just feels so granny-like

So what do you think of this selection? If you can help in anyway I would appreciate it : )

Thank you for reading,

Hana

PS. Please tell me if I forgot anything or made any mistakes - this is my first time posting here