OK new post to not get lost in the original: is it possible that I can NOT upgrade the 486SX 33 without an overdrive slot on the motherboard? After a lot of digging, I seem to find that the 486DX33 can be upgraded to the 486DX2 66 but the SX 33 only accepts the 169 pin ODP CPU which is for overdrive sockets and not for the main socket from what I can read. Basically, I cannot upgrade my PC any further with the current motherboard - can someone confirm this?

I believe the extra pin in the ODP variant is used to disable an existing CPU that comes soldered to the board. So this shouldn't apply on your case as you just have a CPU socket and you are actually replacing the CPU (an ODPR should work just fine there), unless the Aptiva motherboard for some reason requires that extra pin to be there...

Yeah this is what I understood as well. I just never saw any mention of a standard socket having 168 pins. Everywhere I read, it said the Overdrive socket had 169 pins to avoid putting in the CPU the wrong way so I was surprised to see the main socket having 169 pins and wondered if it was some kind of weird hybrid.

EDIT: anyway, I replaced the old 486SX and it's working fine again. I don't want to risk breaking the motherboard though so I think I'll just keep it as it is and use the HP Vectra Pentium 75 instead to bridge the gap. I just need to get hold of an AWE64 to put inside of it but I got a Sound Blaster 16 for now.

Sorry to respawn such an old topic but out of curiosity sake:
My aptiva has the same mainboard as yours and I can confirm that you can upgrade it using a regular DX or DX2 CPU.
Mine is using Intel DX2 66MHz and it's running just fine.
The problem is with the DX4 CPUs which it shouldn't support because it lacks a voltage regulator (DX4 is 3.3V against 5V of standard DX2).
IBM didn't made clear if this board are supposed to take only ODPR or if it would take ODP CPUs as well and yeah, it is quite odd that they seem to support the 169 pin version (ODP) while using just one socket (some documentations stated that they should support even the 487DX Coprocessor).
As some folks might know, the 487 CPU wasn't any standard math co-processor. It was indeed a CPU + FPU on the same package and when inserted, it should disable the CPU (that's why they had a 169˚ pin) and use only the 487, so it was kind of a Intel DX with different pinage.
That's why 487s also MIGHT work with this board.
Your Overdrive DX2 66 "ODPR" should have worked, but they put explicitly "ODP" on the manual. So as you said yours won't work, our mainboards might just take the ODP CPUs instead (and this is odd as hell, since ODP CPUs were meant to be used with a secondary socket without the need to remove the main CPU and our board doesn't have any secondary socket).

Well, I ordered an ODP Intel Overdrive DX4.
We will get to know the truth about the aptiva very soon.

Yeah, it worked flawlessy.
So yeah, you have to use 169-pin ODP Intel Overdrive, even that the motherboard has only one CPU socket.
Kinda odd, but it works.