Any sand filter will be better than the standard Intex paper cartridge filters. Larger sand filters are better than smaller sand filters, though at some point the filter is big enough and larger still doesn't gain you anything. A 50 sqft cartridge filter is roughly equal to a 21 inch sand filter. Cartridge filters have pros and cons compared to sand filters. You might want to read the article on different kinds of filters in Pool School.
While often sold together, filters and pumps are two separate components. Typically you want the largest filter you can afford and have room for. However, later pumps draw more electricity, which can get to be quite expensive for very large pumps so things aren't as straightforward with pumps. For pumps, you generally want 2500 gph or better. However once you go above that it is worth looking at two speed pumps as they can save you a great deal on electricity.
For your setup, the very minimum I would recommend is the larger Intex sand filter. Moving up to something fancier than that is well worth it, but more expensive and may or may not be in your budget.