I was able to run Arma 2 (DayZ I'm guessing?) on a Radeon 5570 on low settings. I guess if you turned the graphics down to the lowest it goes you might be able to pull it off.
The 6450 should fit also.
The requirements are as follows:
OS: Windows XP or Windows Vista
Processor: Dual Core CPU (Intel Pentium 4 3.0 GHz, Intel Core 2.0 GHz, AMD Athlon 3200+ or faster)
Memory: 1 GB RAM
Graphics: GPU (Nvidia Geforce 7800 / ATI Radeon 1800 or faster) with Shader Model 3 and 256 MB VRAM
Hard Drive: 10 GB free HDD space
Here you can see where those cards are on the chart:http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
The 7800GT is as fast as a 6450.