You won't get faster speeds as such, but you will most likely (especially on 1.21) get a much steadier throughput because the thing is designed to meet the rigours of much faster streams. Basically it will probably beat your old G router if you just have it on G. It has a great feature set and the QoS seems to work well. All things combined I think you will get a steadier connection, especially if you are streaming movies and stuff.
One of the things that caused me to buy the router is it copes well with lots of connections, has a big data throughput and good features. Until I upgraded from 1.21 it was unbeatable in my book.
Even with current problems and it not being as good as it was, I have it running my 50MB cable connection with a server that I use utorrent on a lot (I can get steady 4GBit downloads on), it streams my TV well and I can watch movies in my bedroom. Gaming is crap right now due to the constant stutters, but maybe they will fix it.
If you're wireless G network works well and does all you want it to though, I can't see much benefit in upgrading.