This is a great question and gets debated a lot.
SSDs do work great in G5.
Here is my info that I recommend you checking out that gives you a sense of the ins and outs of what TRIM does:
http://g5center.net/index.php?post=overprovisioning
My Hardware page has more info too:
http://www.g5center.net/hardware.html
I do use a super cheap Corsair 60GB drive that I got for like $40 from MicroCenter. I am looking to upgrade it, because it's not very fast at all. I get a ton of info from the guys at SomethingAwful who tend to be pretty darn knowledgeable about the best SSDs, and they recommend the Intel 520/530 series for older machines without TRIM support (even XP machines for instance). They have aggressive Sandforce processors which try to clean up and optimize the drives in the background. OWC drives also seem like a good solid pick, even if they are a tad bit more expensive.
The best SSDs have consistent performance. The lower end ones may have an initial blast of good performance, but then they taper off because of a lack of built in memory or poor design. My Crucial falls in that latter category. Samsung EVOs are good, although there was a massive bug that meant older data began to suffer serious performance bugs. The fix was just released. They are also not Sandforce based.
Intel drives tend to not be the highest performers, but they are built for enterprise applications so they are pretty consistent and reasonable in price.
Sandisk Extremes are also solid, and if you can find the Mushkin Deluxe drives, those are solid too.
The new Crucial MX series are probably some of the best priced drives out there, but they don't always have the most consistent performance. For a G5, it's probably good enough though, since you will quickly saturate that SATA I port.
Isn't it nice though that G5s even have SATA ports?