All boils down to what your motherboard can support. Faster is better.
I would advise not to chase "perfect match" dream. Some games, modes or quality options can cripple 3850 even on Prescott and some can be very hungry for raw CPU power.

I own a 775i65g revision 2.03...
I have also lying around a cedar mill with 3.40ghz.. but that's also single core ...

As The Serpent Rider says, boils down to what your motherboard supports.

Experiences from my own Win9x/WinXP build with testing both the HD4650 AGP and HD3850 AGP with a varetity of games is that you'll come to about 2009-2011 in games no matter what. In about that age range games started to become multithreaded and a single core CPU just won't cut it. Also this is about the same time these cards were introduced, but remember they were not highend cards at the time. So running games in 1080p with everything turned to max at 60fps+ is just not going to happen, as you'll be GPU bound (Bulletstorm from 2011 is an example). So if you have a quick P4 that works fine with that card if you limit yourself to 2007-2008 and below (Crysis not included).

ATI drivers always had a lot more of CPU overhead comparing to nvidia. I would suggest to go with high clocked Core2Duo with high IPC.
Core 2 Duo E6600 oc'ed to 3..3.1 GHz will do the job.

I will try and compare the p4 (cedar mill) and a core2duo e6700.. the cooler is a noctua nh-u9b - that should give some room for overclocking...

Best socket 775 CPU for Radeon HD 3850 AGP

Intel® Core™2 Extreme Processor QX9770
https://ark.intel.com/content/www/us/en/ark/p … 00-mhz-fsb.html

Intel® Xeon® Processor X5470
https://ark.intel.com/content/www/us/en/ark/p … 33-mhz-fsb.html

My Mainboard does not support fsb1600... Otherwise this would be a nice catch

Updated my post with Xeon which should support 1333.

duga3 wrote on 2021-02-13, 12:24:

Updated my post with Xeon which should support 1333.

😀... Nice CPU also... But also not supported... 1066mhz fsb max

Lazar81 wrote on 2021-02-13, 12:30:

duga3 wrote on 2021-02-13, 12:24:

Updated my post with Xeon which should support 1333.

😀... Nice CPU also... But also not supported... 1066mhz fsb max

Aargh, that's a pickle. But sometimes getting newer mobo+cpu can be cheaper than getting that one specific/fastest older CPU that your older mobo can support. Not saying that is the case here, but maybe take at least a quick look at other options, outside of focusing on your current mobo. There are often (overlooked) eBay auctions where people sell their obsolete mobo+cpu in bundles for example.

I was using a Q6600 & Asrock 4CoreDual-Sata2 with a 3850 AGp and it worked great. Of course like other said a fast Core 2 Duo will work amazing for sure

My Mainboard does not support fsb1600.

Irrelevant. All Core 2 Extreme have unlocked multiplier.

Q6700 or Q6600 would be my pick. Try to find the G0 iirc stepping if you can, it overclocks better and has better power consumption.

Hello ... I did some research and it looks like the qx9770 and x5470 are not supported by the mainboard... But please - proof me wrong.

I looked further and am wondering if I just should get following CPUs and just see how they perform:
E6700
Q6700
X6800
672
965

--------
Edit:
Just ordered a q6700 ... Will see how it works

If it's any help here are some 3DMark runs from Q6600 & HD 3850 AGP

Just get any decently fast Core2 Duo, even something like a cheap e7500 or e7600 that's rated for FSB 1066 will blow the doors off any P4. What's the point of using a P4 if you board supports Core2?

Games from 2007 - 2010 will run better with a fast dual core, and typically don't benefit from quads.

Lazar81 wrote on 2021-02-13, 12:30:

duga3 wrote on 2021-02-13, 12:24:

Updated my post with Xeon which should support 1333.

😀... Nice CPU also... But also not supported... 1066mhz fsb max

A 3850 was a mid range card in 2008. Any 1066mhz core 2 duo with a decent clock speed will be more than a good match. IMO you don't need to overthink it.

Games didn't start making use of quad cores until well after that card had become completely obsolete. I would suggest an E5000 processor, like the E5800. You won't miss the L2 cache and with its high base multiplier you can easily overclock it to 4.0ghz if you really want to.

Ah well...

thx for the many replies.. every single one is useful for me.

mothergoose729 wrote on 2021-02-14, 16:03:

Lazar81 wrote on 2021-02-13, 12:30:

duga3 wrote on 2021-02-13, 12:24:

Updated my post with Xeon which should support 1333.

😀... Nice CPU also... But also not supported... 1066mhz fsb max

A 3850 was a mid range card in 2008. Any 1066mhz core 2 duo with a decent clock speed will be more than a good match. IMO you don't need to overthink it.

Games didn't start making use of quad cores until well after that card had become completely obsolete. I would suggest an E5000 processor, like the E5800. You won't miss the L2 cache and with its high base multiplier you can easily overclock it to 4.0ghz if you really want to.

The e5800 looks nice to me. Will try that too