Just do it!
I was in the same situation last year. I had a cartridge filter under the skimmer, in the same housing. I constantly had to clean it to keep the flow up. I would need to change it weekly if not sooner. By the end of the season both cartridges were trashed and I had to buy 2 new ones each year. I was spendign 100 dollars a season on filter, constant maintenance and always fighting algea. I opted to go for a sand filter. I got a good deal off an authorized Hayward retailer on Ebay. I purchased a 100 lbs sand filter and a Power Flo pump. I have never had more enjoyment as a pool owner as this year. The water is always crystal clear. I backwashed twice this season, so far. Ever since my upgrade the pool has been more enjoyment and ALOT less maintenance. I strongly dislike cartridge filter due to my experience. I would recommend a properly sized sand filter and pump. I have not added DE to the sand but have been considering it. The ability to backwash and vac to waste is priceless. I am sure if you follow the advice given here at poolforum you will become a happy pool owner and more of a pool user.

Brian