Yea I guess that did sound a little harsh. If he really wanted to see you a bill of goods he would have sold you that $300.00 video card
The differences between the cards are many, architecture, clock timing, hardware, dram timing, and on and on.
Your best bet would be to do a little research. Find some sites that do impartial comparisons.
Let me give you one warning though. If you do put a bigger, better, faster GPU in your machine make sure that the power supply will be adaquate to handle it.
I put a 7600xt in my machine and blew out the psu in about an hour.