Hi there,
With that budget it is a tough call. From experience the 9800GTX (non + version) can play BF3 at low settings on a 1920x1200 resolution and get ~35FPS when it is coupled with a good CPU (I was using an i5 750). Therefore, it is playable with a 9800GTX+ but not ideal - but with this budget it may be the best you can achieve. My suggestion would be to first get the mobo and CPU upgrade, then later aim to spend £100 on a new graphics card like a GTX 460 1GB or HD 6850.
Also, bear in mind that all current systems use DDR3 RAM, so as you are likely using DDR2 RAM then you will need to buy new RAM as well as a motherboard and CPU.
For buying new parts the best option is to go for an
i3 2100 for £96, a
H61 board for £48 and
4GB of DDR3 RAM for £18. Total cost before delivery: £162.
However, personally I would look second hand and pick up an X58 board and a i7 920/930/950 CPU or a P55 board an i5 750/760 CPU. Each of these can be had for around the same price as the i3 2100 cpu and H61 board but these are Quad core CPUs (the i3 is a dual core) and they allow significant overclocking (i3 can't be overclocked). These i7 900 series and i5 700 series CPUs may be last generation parts, but they offer more performance at stock speeds than the i3 2100 and the ability to overclock.
There seem to be a lot of these older high-end boards and CPUs available second hand now as many people have sold their old kit to upgrade to the latest and greatest sandy bridge quad core systems, so the prices are really quite amazing.
