One has a P35 board with a Core2-7300 processor. It has a 4870 card. Not going to upgrade the Video Card.
The other has an Athlon AM2-6000 (board is AM2+ maybe AM3 and limited overclocking capability). This is running a 4850 and was going to upgrade the Video card to something like the 5850 card.
I'd leave the core2 alone, or maybe try to overclock it. Most current games still only use 2 cores, so unless you have a ton of stuff running in the background when gaming you wouldn't see much, if any benefit from a Quad.
If you're running an athlon 6000+ then the board is AM2 or AM2+ The only difference is that the AM2+ boards can run DDR2 1066 RAM, while the AM2 are limited to 800. In either case, you'd be able to plug anything upto the newest P2 965 into the socket, if the motherboard manufacturer has a BIOS that supports it. (unfortunately, if it's an older board a bios update may not exist) The big question for upgrading is 'where do you want to be?' a 6000+ and 4850 system isn't a slouch, and can play most current games just fine at reasonable settings. If the Mobo is cheap it probably isn't worth it, but if it's a high end mobo, then it might be worth it to drop a Phenom II x3 or x4 in there along with a 5850 or 5870.
Lastly, regarding the boards, do either of them support CrossFire? a 4850 and 4870 running in crossfire are probably going to be faster than just a 4870 in most titles.
The reason that the upgrade might be helpful is that the 7300-4870 is playing from a 24" MVA panel with a native resolution of 1900x1200 8ms. (XP32 Home).
The AM2 6000-4850 is running with a 26" 1900x1200 TN panel 5ms. It DOES have a cheap Motherboard but also is running Vista32 Premium with 4gb ram (3 effective). The ugrades are planned through January so no rush on when/what to buy.
its worthless to do anything less than a memory/motherboard/cpu on either.
either keep them how they are or do a full upgrade.
a few things to consider that might be worth it.
socket 775 intel is a dead end and not worth upgrading(maybe score a cheap q6600 and oc it to 3.2-3.4ghz)
the amd is probably AM2-- this isnt really supported on phenom2's as it doesnt have the right hypertransport or voltage sometimes.
If its an AM2+ then it would support a new phenom 2 (with bios update) but you would have to use ddr2 memory.