Here http://www.tomshardware.com/reviews/radeon-geforce-pric... we see a chart or hierarchy of gpu performance. In his explanation of the chart, its top performer on down, and he recommends at least a 3 tier jump for an upgrade, to truly see a perfomance difference, all of which I totally agree. On the chart, the 4850 is 3 tiers up, while the 4870 is 4 tiers. Alot depends on how much of an upgrade youll want to see, and your rigs configuration: resolution/cpu/psu mainly
The pricing on the ATI cards have been reduced dramatically lately, so, if I was in your situation, Id go big provided my cpu can handle/truly show the difference, my psu can handle the card, and my resolution warrants it, or also be willing to change a few other things on my rig as needed, whether it be a oc to my cpu, a more powerful psu, and what size my monitor was
Jaydeejohn's first post about the heirarchy chart and its use was great; use that. If there's a particular game that isn't running too well for you, perhaps there are benchmarks of that game on various cards that would suggest how high you want or need to go.
A quality 450W PSU is enough for a 4850; an Earthwatts 430 could do it.