Hang on, why are you only talking about Pro Logic? Is your receiver old enough to predate Dolby Digital? If so, then that's the biggest reason to upgrade. Dolby Digital will give you vastly better results from DVDs. Pro Logic II will only come in for stereo signals, eg from TV.
And John - most receivers don't let you select Pro Logic IIx in 5.1 mode. They just offer "Pro Logic II". Are you saying that the "Pro Logic II" these receivers offer is better than the "Pro Logic II" of non-IIx receivers?
And ekb - stop saying "DD PLII", it doesn't make sense, assuming you intend "DD" to stand for "Dolby Digital".