Plug in g.a.s. is a disease - be careful about getting sucked up in it. I don't think there's any one plug in that you need that desparately. Voxengo has some awesome plugs that are cheaper and just as good as UAD, in my humble opinion. They're just slightly different flavors. I will say that the UAD Plate 140 is killer, but I'm sure there's alternatives that are cheaper and almost as good. I have the LA2A and it's often my go-to comp for vocal, but I could live without it and be just as happy with something else. I have the demo of the PSP and I love it, I just don't need it.
If you dig the other UAD plugs, get the card. If not, just get the PSP. Check out Voxengo too. You can download full-function demos before you buy.
If cash is an issue, you will get a lot more mileage out of Voxengo. Their stuff sounds great, and you could have a comp (Marquis), limiter (Elephant), MBC (Soniformer), and EQ (Gliss EQ) for about $300.