Not a huge amount of detail to go on, so I'll try to cover all the bases.
Firstly, the UAD Apollos are great interfaces but they will only run plugins which have been designed for them. They come with a number of these, and more can be had from UAD, but you'll need to bear in mind that your current plugins won't run on them. If you don't yet have a pro-level audio interface this'd still be a good choice anyway, but if you want more oomph to run your current plugins, upgrading the PC would be a better bet.
Regarding the PC, you say it's great but you haven't given any details, so we don't know how up to date it is and whether there might be newer, better options open to you. Similarly with Tracktion, we don't know if you've been using the same version all this time or if you've been upgrading as time's gone on. As the newest versions are now named Waveform, could be that you haven't. More recent versions may spread the plugin load over available CPU cores better. Similarly, a newer or better CPU might give you more power to run plugins with.
Finally, there's the least expensive option, which is to render tracks to audio. This is especially pertinent if you have a lot of instrument VSTs in your song; rendering them to audio and removing the instrument plugins will free up the CPU they're using and you'll then have more to spare for effects. Similarly you can render tracks with effects on them to audio and then remove those effects plugins. If you're not making a lot of use of aux/subs/busses, those can also help reduce your plugin count by routing multiple tracks of audio to a single sub which then had one set of plugins on it.
Oh, and a bonus forum hint: it's usually a better idea to start your own topic thread rather than ask a new question in an existing thread, because that way your replies won't get mixed up with replies to the original topic.
I hope that some of this has been helpful, if you want to let us know what your PC specs are I'll probably be able to offer more suggestions in that area.