I'm not sure I get it. Why would you put GDDR4 memory on a card that wouldn't get any benefit from it? Doesn't that just mean that you'll get one card that is just as fast/slow as it always was (the 3850 AGP) and one broken card (the 3870 you scavenge the memory from) ?
This makes no sense to me. If modding a card means you'd get a noticeable performance gain, then sure, but otherwise, it just seems like a waste.

3870 gets most of its speed over 3850 from the core clock. It isn't very bandwidth limited. Note that 4850 has GDDR3 of 3870's speed class!

Plus, GDDR4 may clock a bit higher but it also has higher latencies and so GDDR3 is right on its heels.

And you'd need a BIOS designed for those GDDR4 chips. The board may need changes as well.

I came to the idea really because i wanted to upgrade my Phenom II X4 920 with something faster than the HD4350 that was in, before that i had a 6800GT in and was around same speed but different noise level, not the better one's.
I bought an 7900GT EE for 12euro which showed some artifacts and did a vmem mod to 2,09v and is ok now but need to cool the vrm's with heatsink because it is overheating and the artifacts then are coming back. still have to do this.
So i bought a EN7800GTX to replace it for 7,50 euro's and same thing here, its factory overclocked to 486core and 1350mem seems factory oc nvidia cards are screwed in the end ... so i threw it out, shitty nvidia's (i used to be an nvidiot).
I bought then a HD3870 512MB GDDR4 and 8800GT 512MB for 25 euro together and bcause the 8800GT is faster it is still in the machine now.
But i could not resist to go faster in the end so i ended up buying:
Some few HD4850 one with 512MB DDR3 and one with 1GB DDR3 and some HD4870's 1gb's and some HD4890 BE 's.
I could not resist, the HD4890 is a neat card for few bucks to buy them.
Btw A HD5850 coming my way now for 10 euro's 😉

So came to think of it as i love my other AGP system so much with core2deo inside... could i upgrade the HD3850 AGP? with some more modern HD4850 1gb GDDR3 😈 or maybe GDDR4. just for the awesomeness.

As i work with soldering all day and we have a bga machine at our repair centre at my disposal i tried some things this week.
I have a voodoo3 with bend card, the cpu came loos on a few balls, i tried to reflow but they got joined and could not seperate them again.
After i did some hand job reballing with lead-solder i was thinking to myself, YOU CAN MOVE GDDR4 to your HD3850 AGP. (GDDR is only 136balls x 8 chips is reasonably doable...) after that i thought youre crazy, and your crazy to sacrifice the HD3870,.. but the HD3870 does not seem to be a very valued card after the HD48xx cards came in, actually the HD3870 kinda is sucky...most hate them and i believe most that are not for sale on the internet end up in the daily thrash.

What a waste for the ...... GDDR4...hmmm should i actually speed bin my current 9 HD3850 AGP's and find the best one to move the GDDR4 preferably the 1GB content.

I think if i would succeed its kinda (awesome) ok. and make the core2deo a 5-10 more happy. 😉

But that is just my head for now and its kinda hot out here for some days, maybe i lost it somehow.

Who knows.

Btw indeed i need to change the latencies on the memory profile so i would take out the bios from pcie 3870gddr4 to the agp 3850 and change the gpu voltage to the 3870 gpu version instead of only 1,2v, as the gpu is actually the same chip with 666M transistors. it was meant to be to turn it to a real beast.
Have to find me a card with the fastest 1gb gddr3 (i have a 9800GTX lying in the closed...)or to use the gddr4 route, i would have to move the resistors to pull up the signals gddr4 instead of the gddr3 circuit, pcb should be the same and i can swap over parts.
The heatsink i could move also to the agp version i think because the pcie to agp translator is on the back and seem the same as sapphire put on them.
the HD3870 should then get the 1.2ns 3850 gddr3 ram back and would end up a HD3850 for sale or backup card.
So no waste, only time would be wasted, but they are my heartbeats....

But they are just clouds for now, after my vacation i will finish my handjob voodoo3 (does that sound nasty?) and i will judge if the agp upgrade would be a next project on work.

I think i maybe need to beg at hynix for some samples of their last storage of GDDR3 type H5RS1H23MFR-N3C 1gbit gddr3 at 1,3ghz 2,6Ghz DDR. the time has come to still obtain or buy some fresh binned gddr3 from hynix. while i still can since they are already obsolete for some time now.
A AGP HD3850 with 1GB GDDR3 at 2,6Ghz and a overclocked gpu to 850 would be.... very unique... i guess. more different than having a V5 6000 maybe 😐

Seeing a AGP HD3870 1GB on 83GB/s on memory in GPUz makes my clockwork going crazy.
Using a hires and AA no worries 😀

Seeing GDDR4 datasheet Hyinx at HY5FS123235AFCP-06 part 3,2Ghz, a 512MB at 102GB/s mem throughput just awesome, i do not even have to change the VDD/VDDQ as it is the same as used 2,0v on the board.