Arma 2 Specifications:
MINIMUM
CPU: Dual Core (Intel Pentium 4 3.0GHz, Intel Core 2.0 GHz, AMD Athlon 3200+ or faster)
RAM: 1GB
GPU:(Nvidia GeForce 7800 / ATI Radeon 1800 or faster) with Shader Model 3 and 256 MB VRAM
OS:Windows XP
DVD:Dual Layer compatible
HDD: 10 GB free space
RECOMMENDED
CPU: Quad Core or fast Dual Core CPU (Intel Core 2.8 GHz, AMD Athlon 64 X2 4400+ or faster)
RAM: 2GB
GPU:(Nvidia GeForce 8800GT or ATI Radeon 4850 or faster) with Shader Model 3 and 512 MB or more VRAM
OS:Windows XP or Windows Vista
DVD:Dual Layer compatible
HDD: 10 GB free space
Your CPU, RAM and GPU are below specification requirements. You will certainly have to upgrade your motherboard when you upgrade those components which may also require a new PSU to handle the increase in voltage. In short, you will have to purchase a new PC in order to play Arma2 as well as many other game titles these days. Sorry pal.