There really isn't anything in circulation to look out for. $20 bills seem to have become "ATM coupons" and the supply turns over so quickly that even the previous black and green large-portrait designs are hard to find. It's been years since I've run into one of the older small-portrait bills so they're effectively gone from the average wallet.
That said, there are some nice ones to look for on auction sites and at currency shows. These are generally available at prices that aren't too outrageous:
1950-E FRN; ~$55 circulated, around $100 uncirculated
1934 FRN Hawaii overprint; $75-$125 circ
1934-A FRN Hawaii overprint; $45-$75 circ
1929 National Currency; $25-$125 circ. depending on the issuing district
1928 FRN; $22-$500 circ. depending on the series letter and district
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There are several books on collecting money that you can probably find in your local library or large chain bookstore. This should answer your question.
Yes, anything can be a collectable collectable just means you can collect it, and we all get bills