1flomar7 :
huilun02 :
If that's what you currently have, then there's little point getting an upgrade. Two 7870 could probably match or beat a R9 290. Meaning an upgrade is going to be really expensive if it is to outperform what you currently have.
little point getting an upgrade? im running at 150 fps on lowest possible settings with chris maxframes config and still getting fps drops while other people play max settings 300 fps no drops
People who tell you their fps are most likely lying if it is a realy good fps. I meet tons of people on games like Arma 2 and BF3 who tell me that their FPS is above 200 on max settings. While yes, this is possible with Xfire 7970s or maybe 770s, it's definitely not plausible for all these people you run into to have the expensive GPUs. However, when i comes to an upgrade, I would go with an r9 280x. This GPU should be able to max out TF2 and even go beyond your monitor's refresh rate. I myself also run a 7800 series card I I currently don't have any problems with it but plan on upgrading to that exact card soon. Before you buy anything though, make sure you have the latest drivers installed and try tweaking the settings and see if that will make a difference. There are also many forums around the net that will tell you the best way to get better FPS and picture quality from a game. I wish you luck!
Edit: BTW if you said you can spend up to $800, try looking into a cpu upgrade. WHile this is not necessary, it definitely help in the long run. If you don't plan on overclocking, you can cut the costs by purchasing a lower end motherboard. Also, before you buy the GPU, make sure that you have a strong enough power supply and that it can supply enough power for a new GPU.