All ROMs, Kernels, Tweaks are almost always free (actually I think it's illegal to charge for them, with android being open source and all. I haven't looked through the legal aspects yet, so I can't be sure.)
Go with flashing ROMs first, they'll be the most obvious changes to your phone. They usually come pre-tweaked as well, so you won't have to bother with those, although if you want to, there are a large amount of them that are usually ROM specific- make sure you have the right one. As you get more confident, go ahead with kernels and radios.
I've found that CM ROMs usually are faster and have amazing batter life. The trade off is that they're very sparse.
Sense ROMs are very powerful, but they have average battery life and can be slow. They do have a lot of pre installed gadgets that often times you can't get on the market, and they require HTC's framework to operate (meaning they only com with, and work on Sense ROMs). The main reason I use a sense ROM is because of the awesome camera.
There are plenty of other ROMs out there, I've just never used them enough.
Also, check out Desire HD ROMs as well. Both are nearly identical phones (everything is the same, the I4g is just 4g capable, the DHD can only use 3g), and ROMs will generally be made for both phones. Sometimes the DHD forums will have a few ROMs not shown on the I4g forums.
BTW: everything is just my opinion and based on what works on my phone, and my phone alone. YMMV