new pump needed

[PoolDoc]

OK:
=> 20,000 gallons
=> 1.5" pump return from skimmer;
=> (2) 1.5" pool returns from filter

Your maximum effective flow rate is set by your single skimmer return -- you do NOT want to exceed 6 fps (ft per second) flow rate if possible, because pumping rapidly becomes in efficient as you do so. (page 47 of the Charlotte Plastic Pipe tech manual) This limits you to 35 - 40 GPM. That's a little low for a 20K pool -- 42 GPM is 8 hour turnover (= a volume equivalent to your pools volume goes through the filter in 8 hours, but that does NOT mean that 100% of your pool water is filtered in 8 hours!)

So, your looking for 35 GPM, but need better water quality than you'd get with a sand filter on 9 hour turn-over.

Assuming you have no other equipment, you could get excellent energy efficiency with a 2 speed 3/4 HP Whisperflo
model WFDS-3 for about $570 + shipping, OR a IntelliFlo Variable Speed . . . which is just a 3HP Whisperflo wet-end with a variable speed motor. Considering that you'd have to install a special timer for the Whisperflo, the Intelliflo might be a better idea.

I don't know what a "175 cartridge" filter is, so I can't comment on whether that's a suitable filter, or not.

However, if you wanted optimal water quality, something like a Hayward ProGrid 60 or even 72 would give you that:
http://www.a1poolparts.com/-strse-25...ter/Detail.bok

If you select a cartridge filter (other than the one you have now) remember to check on replacement cartridge cost BEFORE you buy the filters. Some modern cartridge filters, like the StaRite System3 have EXTREMELY expensive replacement cartridges, that are also very difficult to handle when wet and dirty.