My office system is an old Pentium 4/1.8Ghz CPU (not HT enabled), with 1GB of RAM, and an old ATI Radeon 8500 w/128MB of video RAM.
It plays the game very smoothly and only stutters a bit when a lot of aircraft are in the area.
But, I have set the game up to run well on it. Single-threaded CPU's would do well to use 256 or 512 maximum texture sizes. If you have a 1GB of system RAM, or higher, go ahead and preload the textures to RAM.
I also disable skins. It helps to reduce texture swapping, which is important on single-threaded CPU's.
Sound cards are another high resource demand. Using sound packs which have huge sound files can really hurt your performance and induce enormous stutters. Particularly if you use an onboard sound chip, or a low end PCI card (SBLive! or less). AC97 sound device users should never load up optional sound packs. AC97 devices are 100% dependent on the CPU.
I run into a lot of players who have significantly faster systems, than anyone at HTC, who have terrible performance. Most of the time, they over load thier resources. Too high of texture format, big sound packs, far too many background processes, anti-aliasing enabled on the video card, and other resource consuming issues.
Cleaning up the background processes is a free performance gain for everyone. If you have more than 21 processes running in the background, then you are losing a percentage of your CPU performance.
It does not matter how good your video card is IF your CPU cannot get the data to the video card.