No you need a better CPU to play it at all and a better card than a 6450 without lag. As your board can only take AM2+ CPUs you would be better buying a new PC than trying to upgrade unless you can find a cheap AM2+ CPU on Ebay like a Phenom 940, if you go that route then also look out for a used Radeon 4850 or 4870. If you can get these for under $100 for both then get a new 500W PSU for around $50 and you will be able to play games on low-medium settings at a good resolution. Alternatively find $400+ (not including windows, monitor, Kboard & mouse) for a new build.
Even if you had a better PSU the 6450 is one of the worst cards you put in for a gaming card. I will link you the hierarchy of GPU's so you can see how far down the chain it goes.
Here are the system requirements for Arma 2
Minimal PC Requirements
CPU: Dual Core Intel Pentium 4 3.0 GHz / Intel Core 2.0 GHz / AMD Athlon 3200+ or faster RAM: 1 GB Video Card: NVIDIA GeForce 7800 / ATI Radeon 1800 with Shader Model 3 and 256 MB VRAM or faster OS: Windows XP
Optimal PC Requirements
CPU: Intel Core 2.8 GHz / AMD Athlon 64 X2 4400+ or faster RAM: 2 GB Video Card: NVIDIA GeForce 8800GT / ATI Radeon 4850 with Shader Model 3 and 512 MB VRAM or faster OS: Windows XP or Vista