It depends how much money you're willing to spend. One of the best voices you're going to find for free is the L&H TTS 3000 "British Female" There are several companies you can find with better voices -the best I've heard is from a company called "nuance". More realistic, which I what I think you're getting at when you say 'sexy', voices can cost from $20 up through hundres of dollars.
You also need to consider that it's not just the tamber (vocal quality) of the voice that effects how sexy it sounds, it's also timing, inflection and emphisis. Most of that is far to complex to have a computer program change the inflection for each word and the context of the word within a sentence, or within a conversation.
If you want to hear a really, really sexy voice - you're still going to have to be nice to a real girl. If you want to hear a better voice than your current options you're going to have to shell out some cash.
You can always use the links from the Zabaware "Plug-In" menu, or from the MSAgent ring, BotSpot or just do a google search for "TTS" or "Text To Speach"
Good Luck
WholyChao