Despite all the tales of doom and despair you read on these boards, the vast majority of drives do not fail even from the worst lemon products. If your drives have made it this long, there is nothing to worry about as far what was killing them off years ago. I'm amazed at people still posting about one of these drives dying on them now and acting all pissed off that they got bitten by faulty 75GXP bug. No you didn't, these drive were released 6 years ago, they're 3 years removed from being under warranty, and have even exceeded their designed service life. All drives will die eventually.
If you have important data on those drives, I would absolutely recommend replacing them. Not because they are 75GXP's, but because they have exceeded the 5 year service life that all ATA drives are designed for, which means the failure rates quoted by the manufacturer and all other claims for reliability are no longer valid. Also, you'll obviously get no warranty replacement if the drives die. Any current non-Seagate drive will be repectably faster. And if you browse the hot deal forum, you'll find deals all the time for 250GB-300GB drives for well under $100 after rebate. At those prices, there really is no good reason not replace drives that old, unless you truly don't care about the data on them.