Although it is debated whether the ear can hear quality past 128 kbps. If your not an audiophile, your current library is fine. Personally I can here a difference between the standard 128 kbps and the iTunes Plus 256kbps. (especially when I am listening to songs over TOSLINK to my audio receiver. PLaying it very very loudly can cause a rift, I have compared songs and noticed the percussion more pronouced etc.)
Bottom line with this economy, I don't think it's worth it to upgrade ALL of your songs...just buy iTunes Plus with any new stuff. Or buy CDs and rip into Apple Lossless ;]