512 MB DIMMS must have 256 Mbit technology devices

It has to do with the size and width (how many bits) of the memory chips used to make the DIMM. A double sided DIMM arranged 32x8 would work. You'll need to read the memory specs to determine if yours are made properly for your application.

If your 512MB DIMMs do not have 8 memory chips on each of the two sides, they are not likely to work.

If they are fast enough, and fit easily in the slots, and you try them, they will either be:
• not recognized at all
• recognized at half their advertised size
• recognized at full size and work fine

There are many different memory parameters that are important. You need to:
A) become an expert on all of these different parameters. -OR-
B) purchase memory from someone who IS an expert on memory for this Macintosh.

• Do not buy memory from a department store or an electronics store. Buy only from someone who sells memory for Macintosh. This rewards vendors who have cultivated Mac expertise.

• You should tell the seller what Macintosh you have, and the seller should send you the correct memory for your Macintosh, AND guarantee that it will work in your Macintosh or he will refund your money. (This encourages the seller to send you the right memory the first time.)

• The seller should guarantee that the memory will continue to work for a very long time, or he will exchange it for a new DIMM. (This encourages the seller to send you only high-quality memory.)

The replies you've got so far seem OK, but you're probably asking what exactly does "256 Mbit technology devices" refer to?

At least, that's what I'd like to know.

256 MBit technology refers to the manufacturing process.
Unless you are a semiconductor engineer, there really is no need to explain beyond that.

You can find all kinds of explanations, though.
If you join JEDEC, you may find many helpful documents regarding the standards of the production processes.
Other info is proprietary, and available from the likes of Micron, Hitachi and Samsung.

What you need to know (as a user/purchaser) about the RAM is this:

The RAM expansion slots accept 168-pin SDRAM DIMMs that are 3.3 volt, unbuffered, 8-byte, nonparity, and PC133 compliant.

DIMMs with any of the following features are not supported in the Power Mac G4: registers or buffers, PLLs, ECC, parity, or EDO RAM.

If you are a developer, you need to know this:

The mechanical design of the SDRAM DIMM is defined by the JEDEC MO-161-D specification.

The electrical design of the SDRAM DIMM is defined by the JEDEC standard 21-C specification.>The presence detect serial EEPROM specified in the JEDEC standard is required and must be set to properly define the DIMM configuration. Details about the required values for each byte on presence detect EEPROM can be found in sections 4.5.4 and 4.1.2.5 of the JEDEC standard 21-C specification.

Capacitance of the data lines must be kept to a minimum. Individual DRAM devices should have a pin capacitance of not more than 5 pF on each data pin.

If you are trying to figure out acceptible density of individual SDRAM chips on the DIMM, the following table will help:
RAM Addressing

So, as you can see, following the advice given by others, and buying only quality Guaranteed Mac Compatible RAM from trusted Mac specialty suppliers is the easiest way to guarantee success.