Okay, I've done some rather extensive research on this subject...
First of all, the SoundBlaster Live! family of cards and the SoundBlaster 512 PCI use the same sound processing chip: the EMU10K1 processor. Because the Live! family of cards are all the same, I would suspect that you could get a Live! Value about as cheaply as a PCI512, and the Live! Value will work better for you.
As for the Platinum version of the Live!, well... I've got an original Platinum (not a 5.1) that I bought when it first came out. It's nice to have the connectors on the front with the built-in Live!Drive, but if that's not a big deal to you, go with the Live! Value instead of the Platinum.
A word about the "5.1" versions of these cards... There's no reason to buy the 5.1 if you have speakers that have built-in Dolby 5.1 decoding. In fact, the only reason to buy the 5.1 version of the Live! is if you have a set of 5.1 speakers that don't have built-in Dolby decoding, such as the Cambridge Soundworks DTT2200 (note: The DTT2500 and DTT3500 models DO have built-in decoding). The DTT2200 speakers are the only set of 5.1 speakers I know of that DON'T support decoding, so more likely than not you're not going to want a 5.1 Live! card.
Hope this helps.