PostgreSQL is the next PostgreSQL.
(yes I know it doesn't "perform" as well as the other NoSQL stores but performance is not without tradeoffs [2])
[1] http://bret.appspot.com/entry/how-friendfeed-uses-mysql
[2] http://www.caradvice.com.au/wp-content/uploads/2006/12/Ferra...
I'd rather partition the data based on function onto distinct clusters, you know like eBay do.
You can update the indexes in a transaction too, so that's not necessarily an issue. MySQL has problems with this due to locking but the MVCC implementation in PostgreSQL allows much better concurrency.