You are wrong. The M2N-E is an AM2 motherboard and the Phenom II X4 955 is a socket AM3 CPU.
Maybe you could tip me about an upgrade then? I've recently searched for AM2/AM2+ CPUs, but they all seem to be gone from the sites that you buy them from (atleast in sweden). If I need to purchase a new motherboard, please tip me of one that covers the most of todays requirements, again a good price ofc
We need more information on the motherboard. Look at this link, there are 2 M2N-E boards. One is an M2N-E and an M2N-E SLI. The SLI uses the nForce 500 SLI chipset and does not support the newer chips. However, the non SLI uses the nForce 570 Ultra chipset and DOES, just update the BIOS. Look at the CPU support list for your board.
As far as what your PC can handle. a GTS 250 is basically a physically smaller 9800GTX. Your CPU is what may hold you back a bit, but it will still run. BC2 likes quad core CPU's, however it runs fine on a dual core. I borrowed an 8800GTS at the moment while my 5870 is off on an RMA replacement and I am able to get 60FPS on a 1680x1050 screen with the settings on high, HBOA off, AA x1, AF x1, Vsync ON. This is in Vista so it's DX10 and honestly compared to the 5870 it STILL looks good on the 8800GTS. However, I am also running this on a 4GHz i7. So you are going to take the CPU hit. I would get the game, it's not going to run bad at all. It scales pretty darn well across different systems.
I am actually able to run it on my parents laptop which is a T6600 dual core and 4650 mobile. This is everything on low and off in DX9. I get about 50 FPS.