Quote - Since I'll be in the market for a new PC next year I'm wondering if they sell a MB that will handle more than 8-16GB of memory? Reason I'm asking is because it will very likely be the last computer I will buy for a very long time.....I'm looking at the price of a Phenon II X6 MB, and probably at a budget of around $300, starting out with about 4-8 GB of memory....

I would recommend just saving up money and plan on spending more than $300.  $300 seems to be shooting for the bottom of the barrel especially since you are talking about buying a computer next year and the fact that you want this computer to last much longer tells me that you'll be spending more than $300 for a motherboard and memory.  There are motherboards today that support 24 GB. That are motherboards that are around the $380 - $400 range.

The thing I would be concern with is what the Processors the board supports and the type of RAM that it supports.  This will give you an idea of how how far you'll be able to upgrade the processor as well as the RAM. 

You can also check stores for refurbished deals that will allow you to buy complete PC that is more powerful than many of the new computers for much less. In my case I was able to buy a computer for cheaper than it would have been for me to buy a motherboard a new processor and new RAM sticks.  The total cost for my computer was about $800 but only because I purchased the extended warranty.  I think it was originally $650+  My processor is an i7 which cost around $300 back then.

Just look around and keep your eyes open.  Once the new tech comes out, you'll be able to find really good deals on some really good computers parts as well as complete computers.