Well there's more to it than just interlaced vs progressive when it comes to DVD player performance, but it's very hard to answer that question without guessing really - only a side by side demo would give you a definitive answer.
IMO though, I doubt you'd get much (if any) mileage in terms of picture improvements from a budget PS DVD player - the X1 has the well regarded Faroudja DCDi onboard already, and while a bit long in the tooth now, your Sony was a very highly regarded DVD player in it's day (and while things move pretty quick, they'd already sussed how to make decent DVD players by 2000)
Even if you get a PAL PS capable player, the PJ will still have to descale it anyway for PAL discs (not a problem if your collection is all R1 though

)
Of course, if you want DVD-R capability, then there's a ready made excuse for an upgrade if you need one
