Depends on your interface (PCI-E, PCI, or AGP) and your budget, but assuming it's PCI-E x8, with a 300 watt limitation, I would go with a GTX 430. It's cheap (about between $55 and $70), has way more VRAM than Arma 2 requires, and should run fine on 300 watts. BUT, what you want to check is how many amps are on the +12V rail. The 430 I was looking at required 22 amps and 300 watts.
Personally, you say it's "good" but it's actually quite old. The HD3200 fell near the bottom of Tom's heirarchy chart and it's difficult to find a graphics card that requires less than 350+ watts. I would consider investing in a new PC soon if you can manage it. If not, look in to upgrading your PSU (might be harder with a Dell - find a Dell Replacement PSU as the voltages may be different) and then upgrade to a faster graphics card and add some RAM (depending on your OS, I would go with 4GB minimum and up to 8GB if you have 64bit Windows).