I'd keep the card for sure. It's still a good card; you'd have to spend a lot of money on speakers to make buying a better sound card worthwhile. The quality of the speakers or headphones being used is far more important than the sound card.
As for possible speaker upgrade, really depends what you are after. I agree that small satellite speakers found in cheaper 5.1 speaker sets won't be comparable to the T40. I can see what justalex81 is saying that any bass won't be coming from the satellites, however, they will lack midrange, which I'm guessing is what you are referring to. Again though, it depends what you use them for. For gaming and watching movies, it doesn't matter so much, but for music, it's like someone cut a big hole in the sound. Z5500 are very good 5.1 speakers though, and are certainly worth considering if you want surround speakers for gaming and watching movies. If music listening is the priority, then far better to stick with stereo. I certainly wouldn't bother trying to cobble up a 5.1 system though, using different sets of speakers. Best to go one way or the other.
With £150, there are a few options. You could go down the second hand route, and get a decent amplifier and a pair of bookshelf/standmount speakers. There are some very good bargains to be had if you do some searching, and you'll get more for your money that way. Some people don't like buying second though, or can't be bothered to trawl through sites looking for a good bargain. If you'd rather buy new, then it's certainly possible with £150.