I am pretty wary of mass produced PCs, they typically use fairly lower quality parts as Blairgowrie mentioned. However, they do buy all their parts in bulk and typically get things at a much lower price than what you are able to buy them for, even over the web. Also, they typically don't make very much money on the hardware, but make their profit by bundling in all sorts of third party crap that you can uninstall, extended warranties and service fees, and Microsoft even pays them for every copy of windows that they ship (or so I hear)
A couple of things: check the power supply, if it is less than 500 watts, you may have trouble with running a good video card with it. Check the motherboard brand, is it a no-name board? Or is it a brand like ASUS, DFI, Gigabyte? Make sure the case uses the ATX form factor standard, not something else. Also, with 3 gigs of memory it is likely running 1GB dimm + 2 GB dimm, so the memory won't be running in dual channel mode. It comes with Vista, so you may have some fun getting TrackIR+joysticks+IL-2 working on it (or it may all work perfectly, who knows!)
All-in-all, this bundle is a pretty good deal, and seems like it could be a decent computer with a new graphics card. If you are hot-to-trot on a new rig, this will probably suit you fine (if you get a good video card). Keep in mind, you won't be getting the performance of building it with better parts yourself, but that is to be expected by saving a few hundred bucks. I think the general consensus you will find among the PC hardware enthusiasts you ask (such as myself and Blairgowrie), building with parts you put together will give you more performance and better longevity. However, if you have never built a PC from scratch, buying a bundle like this and replacing the parts as needed may be a better option than buying all the parts for your own build, and making a costly mistake while mounting the CPU.
Just like buying your the parts individually, the key here is to do your research and be patient! Find out the parts its built with, google them, look for reviews (I tend to not look for reviews, as they are always glowingly positive, but google for terms like "Gateway AMD X2 6000 unable to boot" or, "Gateway AMD 6000 problem", where you find posts by users experiencing trouble rather than reviews that the company PR machine paid to have put in PC World)
Good luck! Stingray
Edit: oh I forgot to add, with a powerful enough video card, it should get you to "perfect" graphics. Getting to perfect with IL-2 typically doesn't take that much actually, the game was written so many years ago that the graphics it uses are kind of dated. Playing perfect at a high frame rate on some of the super detailed custom maps I have seen... well, that remains to be seen. Just keep in mind the screen resolution you are planning on running, the higher the resolution, the more powerful of a graphics card and computer you will need. Running 1024x768? No problem. Running 1280x1024, this computer should be fine. Running widescreen 1600x1000 maybe starting to push this a little bit, running 1920x1200 may not be do-able with good frame rates, but again, depends on the video card you choose.