If you're thinking of buying a PlayStation 3 in the post-Thanksgiving rush, you may notice that it's available in several different packages. You may think, "Ah, the system's the same. It's the pack-in game/movie that's different."

You would be wrong.

Here's a brief history/overview that will hopefully point you towards the best one for you.

When it was launced in November 2006, the PS3 was available in two different configurations: one with a 20 GB hard drive for $500, and one with a 60 GB hard drive for $600.

The differences went beyond the size of the hard drive. While the 60 was able to connect to your router via Wi-Fi, the 20 could not. (Both could be connected with standard CAT-5 cable.) The 20 was also missing the SD, Memory Stick Duo and CompactFlash memory card slots that are hidden on the front of the 60, though both units had the same four USB inputs.

Backwards compatibility with PS1 and PS2 software was nearly 100 percent for both models, thanks to the presence of a chip containg the PS2 CPU (aka the Emotion Engine) and graphics procesor (GPU). Some games had glitches, but they were quite rare. The 'Guitar Hero' games, as well as most games that require special controllers, are still pretty much unplayable to this day. The contents of PS2 and PS1 memory cards could be imported onto the PS3 hard drive via a special dongle reader that plugged into one of the USB slots. The PS3 would create virtual versions of the older cards, allowing you to save as you played.

Despite its being $100 cheaper, the 20 GB was the less popular of the two models. Sony officially discontinued the low-end model in late winter/early spring, though the supply of the 20 had been dwindling to a trickle for a few months prior.

The 60's time as the king of the PS3 world was short-lived, thanks to two announcements Sony made in the summer. A $100 price cut was announced in July, dropping the 60 to $500, the price of the old 20 GB. What seemed to be a good thing turned out to be more complicated when Sony announced that the 60 was being discontinued and the price cut was just its way of clearing the remaining inventory. Taking its place in August would be a new 80 GB PS3, bundled with a copy of 'MotorStorm' and retailing for $600.

As with the earlier two versions, the differences in the 80 were more than just cosmetic. As a means to reducing the cost of production, Sony removed the PS2 CPU chip while leaving the GPU. The CPU would now be emulated by the PS3 hardware, which would spit out instructions to the GPU. While it doesn't allow for as full of backwards compatibility as having the actual CPU, it did work with most PS2 games. The 80 GB did have the Wi-Fi capability, as well as the standard four USB slots and SD/Memory Stick Duo/CompactFlash slots.

Near the end of summer, rumors of yet another PS3 model, this one with a 40 GB hard drive, began to surface. This one would be meant to take the place of the old 20 GB unit as the "entry level" PS3, with a price of $400.

Cutting production costs yet again, Sony halved the number of USB inputs, cutting it to two from four. More importantly, it removed the PS2 GPU, making backwards compatibility with PS2 games very, very difficult, if not impossible. (Backwards compatibility with PS1 games remains, thanks to the the powerful PS3 being able to emulate it entirely.) Also disappearing were the memory card slots.

With the introduction of the 40 GB unit, Sony also started selling the 80 GB PS3 without the bundled-in 'MotorStorm' for $500, though the company has said that it's keeping its options relating to future bundles open.

So, as you go forth to spend, here are the four models you could possibly find:

• The 20 GB, near 100-percent backwards compatibility with PS1/PS2, no Wi-Fi, 4 USB slots, no SD/Memory Stick/CompactFlash slots. If you find one of these, it will most likely be at a used shop.

• The 60 GB, near 100-percent backwards compatibility with PS1/PS2, Wi-Fi, 4 USB slots, SD/Memory Stick/CompactFlash slots. Sony says it has no more in the warehouse, but you may stumble across one at an out-of-the-way retailer.

• The 80 GB, either bundled with 'MotorStorm' or unbundled. About 75 percent backwards compatibility with PS2, Wi-Fi, 4 USB slots, SD/Memory Stick/Compact Flash slots. The bundle is supposedly over, but they're still quite common at larger retailers.

• The 40 GB. Zero percent backwards compatibility with the PS2, Wi-Fi, 2 USB slots, no SD/Memory Stick/CompactFlash slots. This is the one Sony's really pushing this holiday. (Notice most PS3 commericials end with "Now only $399.")

Keep in mind that Sony did do something incredibly smart, at least from a consumer point-of-view: Instead of using a proprietary hard drive format, such as used by the Xbox 360 that can be made and sold only by Microsoft, the PS3 uses a standard 2.5-inch SATA laptop drive, preferably spinning at 5400 RPM. You can find them online or at computer stores or electronics retailers, and turning your PS3 into a 250 GB monster is so simple Sony even tells you how to do it in the PS3 manual.