I just recently bought the HIS HD4650 on Newegg for $49.99 with free shipping. The card was that price for a couple of weeks, but it didn't have the free shipping. As soon as it was free, I jumped on the deal. I upgraded from an Nvidia 7600GS.
I knew what I was looking for, and when the price was right, I jumped on it. I was also considering the 9500GT, but it all came down to the FINAL price and the 4650 was cheaper with better performance. I don't like to deal with MIR's or shipping or anything like that.
For me, my max price I wanted to pay was $50, and I just waited until the right thing came along to fit the bill. To some people, a few extra dollars is no big deal and they would rather get the $100 card rather than the $75 card, others would rather wait for a good deal or forfeit the extra performance for the max price they set for themselves.
As far as the OP goes, if $75 is the absolute max, I think an HD 4670 would be the best bet if you can wait and find a great 1 day deal or something. Otherwise, depending on what your computer specs are or if you would actually rather spend a little less, the HD 4650 or 9500GT would be a great fit. Sometimes you can find good deals on higher end cards, but for the most part, those 2 cards are going to give you the best performance for what you want to spend.
***EDIT***
Also, the HD 3870 and 9600GT would definetely be the best performance, but be at the tip top of your budget AND require a beefier power supply. The HD 4650 DDR4 or a 4670 will give you awesome performance for cheaper, but also doesn't require the separate power so you can save some money on the PSU as well.