Can I replace my internal drive with a 320+ GB memory stick instead (MacBook Pro 13" c. 2009) I'm currently only able to see my internal HDs for my MacBook Pro 13 c. 2009 by attaching them to a USB adapter cable external to my laptop.  Drives are great - but either logic board or something else is causing internal failure problem. I don't want to spend any more money in repairs or replace just yet.  So my question:

Can I just use a 320-500 GB USB memory stick to replace my internal drives?  Are there any downsides like speed, reliability, etc. I know it's a pain to walk around with a stick poking out of my USB port but I am seeing 500 GB sticks for under $50 as opposed to shelling out the big bucks to repair the logic board or buy a new Macbook or Air.  Any thoughts?
Yes, you can do this, provided your system will successfully boot from an external volume still. Failing logic boards don't always do this. Each time you boot you will need to hold the Option key down.

The speed will be noticeably slower, maybe even painfully so. The internal drive connects at 3000Mb/s. A USB 2.0 stick will connect at a peak speed of 480Mb/s with regular throughput being much slower. 

The largest USB stick you're likely to find is 32GB or 64GB. A 320GB+ drive will be a full portable external hard drive, much bigger than a simple USB stick. And bear in mind above all else that this is not a long-term solution. Your logic board will only get worse over time, not better.