Casino Verite is many orders of magnitude faster than Blackjack678. The latest versions of CVData and CVCX allow you to specify the number of threads to use for a sim. Running a sim with two threads takes full advantage of a dual core processor. All Casino Verite products are Vista compatible.
I currently use CVData for sims and CVBJ for practice. CVData can sim virtually any set of blackjack rules, playing strategies, and betting ramps you want. It can also generate index numbers for any counting system. CVBJ (also Vista compatible) is as realistic as you can get. I haven't used CVCX much, but I know that it allows you to set up multiple threads for sims, and it also allows you to derive optimal betting ramps.
Franz is right on all counts. In addition, CVBJ makes practice as enjoyable as possible. There's a female assistant who gives verbal explanations of various features of the game and deals the cards (she can be disabled for advanced players who don't need or want this). There are also numerous, varied drills that can make practicing less tedious.