HD 5670 is a better card right now- its got DX11 and eyefinity (though it isn't powerful enough to really do eyefinity well) which the GT240 doesn't have. Its a better upgrade card because it'll give you more longevity for the card itself.
The HD 5670 is definitely the stronger of the two, comparable to the 9800 GT in performance (the GT 240 is more like the 9600 GT). DX11 support is kind of academic with that card though. You really need some horsepower to crank up the AA, AF and other quality settings to fully take advantage of DX11 capabilities.
Just a quick question. I'm also running the m7750n HP with all stock components. I've just ordered the Corsair HX 750 (because I intend to transfer it into my gaming rig when I have the Benjamins for it in the near future) and also because I've ordered a 2 TB Barracuda which according to reviews requires more juice that the 300W puts out.
Now to my question, I'm wondering whether the 5670 is my best bet or the 9800 GT, the Corsair should have plenty of juice for whatever card I throw at it.
My confusion is with mobo compatibility, I've read a few threads where some 'advanced' cards have failed to function on this system.