Recommend a graphics card?

[Pfeil]

A word of warning: Your configuration is generally unsuitable for gaming. You can bolt on parts all you'd like, but it'll never have the performance of an actual gaming rig.

That said, you should be able to make it run IL2.

The single most important part that people always forget, especially with prebuilt PC's is the PSU. As with all mass produced equipment, the cheapest parts with borderline performance are used.
Because of this, the power supply is likely underpowered, and will not accommodate a reasonably powerful graphics card(it'll probably even lack the right connectors).
If you're not sure what wattage(total, though per-rail is also important) your PSU can supply, open up the case and look on the label. If it's any less than 450W, you should replace it.
You should also look for a 6xPCIE power connector, a Gtx660 needs two of those, a Gtx650 one(You can substitute two molexes for a single 6x connector, if that rail supplies enough amps).
Personally I recommend a modular Corsair PSU.

For the graphics card itself, the Nvidia GeForce Gtx660ti is a good midrange card. Though with your slow CPU, a Gtx650ti is probably a better match.

One more potential(and oft forgotten) issue is simply physical space. Performance GPUs are dual slot which can conflict with existing expansion slots, and depending on the length of the card you could have a conflict with your drivecage. Measure before buying.

If you're unsure about any of this, be sure to ask a local professional first. Don't waste your money on the wrong components.
Reading reviews is also sound advice, just make sure you've got the full picture.