How much time would an HD 4850 graphics card last me before needing an upgrade?

Say I have an AMD 6000+ 3.0 GHz cpu, good enough motherboard, ram, etc. and only want to use my rig for moderate gaming (i.e. medium to medium-high settings) and of course daily computing tasks. How long would one 4850 last me before becoming utterly obsolete? Should I spring for a more expensive card? Again I only want moderate gaming, "Ok" graphics are enough, will be playing on a 17" lcd maybe 19" later.