I was wondering the same thing. I've been doing some searching around on the web and it seems 3870 CF will out perform a 4850, but hang below a 4870. If possible get a second 3870 for best price/performance. Otherwise a 4870 is definitely a big step up.
Depends what you're doing with it. If your gaming is just fine, then not worth it, if you're lagging or have everything turned down to play your games, then yes it's a serious bump. If it ain't broke, don't fix it.
I think dirtmountain hit it on the nail - depends on what you're doing with it.
Also, I'm surprised no one has mentioned this yet, but it REALLY depends on the rest of your system. Presumably you won't be bottlenecked elsewhere - in which case a 4870 would be an excellent upgrade over a 3870.
Got the Sapphire HD4870 512MB.
It's heavy, too long and runs hot. Had to remove front intake fan to install the card. The card with the power connectors is wider than an atx motherboard. I don't understand why they didn't place the power connectors on the top of the card.
I can't imagine what the size and weight of the HD4870x2 card will be.
Search the forums. The 48xx series drivers have a fan issue where the fan idles at 10% and doesnt ramp up properly. If you haven't done the fan trick yet your idle temps are probably about 80 and having to remove the front fan in your case isn't gonna help this as you now have decreased airflow in your system. Search around for the fan trick that is done thru a catalyst profile and that should help with the cooling. I have a few profiles setup, 30% for regular use, 40% for multi-tasking and light gpu use, 60% for crossfire gaming. My idles are down to the 50's and max temps are around 80's instead of over 99 like when I first got my 4850.
I'm lucky that my case temperature and cpu temperature have not been affected by removing the front intake fan to install the HD4870.
My HD4870 hasn't gone beyond 80C. When I boot up it's at 70C and after five minutes the card hits 80C and remains at 80 unless there is fast high load it will peak, fan speed ramps up and temp returns to 80C.
Using GPU-Z to monitor temp.
I have an Asus True Power Trio revision B 550W power supply. The HD4870 is so power hungry it's made my cpu and memory overclock unstable. I've had to return to stock cpu and memory speed until I pick up another powersupply. I'm using an X48 motherboard with a QX9650 cpu.
When I can I'm going to get a larger ATX case so I can have the push pull fan system and a higher output powersupply.
I'm satisfied I bought the Sapphire HD4870 video card. But I am disappointed I couldn't get the card edition that comes with the bonus 2GB memory stick, Power Cinema DVD, Cyberlink software and benchmark software.
I did check out the BIOS flash for fan control and making user profiles in CCC. I didn't make any changes since everything seems ok at the moment except for the powersupply that can't manage my system's power demand.
I'm hoping AMD/ATI include a fan control option in the forthcoming 8.8 driver/CCC later this month.