Yup.... do the upgrade if only to get away from that 8 synth limit. You will pick up some few features in X3.
I moved up to X1 for the very same reason.... a few new features, no limit on synths and more than 2 inputs..... not that I use those things even now..... but they are available to me.
As far as adding new instruments..... even MC6 will handle adding tons of new things. (with that pesky 8 synth limit). When I had MC4 as my main DAW, I was buying and picking up a wide variety of synths, samples, and FX and using them all in MC4.
In addition to the sound center packs and the deals cake offers..... look at the Native Instrument stuff as well. They have a bunch of cool synths and sample libraries that are excellent. Pro quality sounds. As a composer, that is the primary thing to consider.... does it sound real?
As you look around, there are more sample libraries and synths than you have money to buy. Some of the better ones are quite pricey and generally out of the financial affordability range of the hobbyist. When you get to the level of scoring Hollywood films, you will be able to afford them.
Keep a watch over in the Software Forum here on Cake. That is where you will find posts about the latest deals and offerings that fellow cakers have discovered. Everything from the deals in the cakewalk store to deals on the most obscure synth no one ever heard of and everything in between. I look in every so often because you never know when, for example, East West will give away a free version of something......yup, I snagged a free working version of E/W when they gave it away, complete with some sweet string samples..... or someone finds a "no-brainer" stupid price deal on some really cool synth or FX...again, I picked up a cool "spacial enhancer" for free from one forumite's posted link, directly from the company that makes it.
You will also learn, from the reviews and comments on certain products, which ones rock your world and which ones are a total waste of time and money.