Well, let's see... my Spanish is a bit rusty, but I think you're asking me what I like to use my machine for, specifically if I use PhotoShop and Illustrator, and you have some similar Macs to mine that you'd like to upgrade... whew! I'm still in the slow-spanish-speaking phase where I have to translate in my head -- it doesn't come naturally yet, but with practice it will...
Anyways, yes, PhotoShop and Illustrator run fine on this machine. I'm not saying they're speed demons -- Illustrator leaves me wanting a little more speed sometimes, but both are VERY usable. Here's some advice: if you have Illustrator, make sure you update it to the very latest version (by searching on versiontracker.com) because the initial release has SEVERE speed problems. I believe the current version is 10.0.3. PhotoShop runs as expected, not blazing fast, but usable.
I'm torn on recommending a 500MHz upgrade to you and telling you to save for a new machine. I didn't see much of an "in-your-face" speed boost by going from 400 to 500MHz, but I can tell there's some "behind-the-scenes" speeding up going on. Filters are faster, encoding music is faster, but as for the UI and "perceived" speed, there's not much increase. Sonnet Tech. has a 700 and 1000MHz ZIF upgrade, but they don't work in the G4 Yikes! machines yet -- only beige G3s, but I email back and forth with a gentleman named Scott there, and he says they're definitely working on getting both those upgrade to work in a G4 Yikes! machine... how long until that is? No one knows...
If you've got the cash to spend, go ahead and upgrade to 500MHz. owcomputing.com has a 500MHz upgrade for around $260, but I went with Sonnet just because I've had good experiences with their products (I also have an ATA/100 card running two more hard drives from them, and it runs flawlessly and fast!).
Next step for me is a G5. I've been saving for a while, and have a while to go, but I think that's the next step -- no more upgrades to this machine.
I'll have to disagree with sebaz on his statement, though... I find OS X MUCH more stable than OS 9, and I would rather do my important stuff in OS X rather than 9 simply because of the "trustworthiness" of it. The speed decrease in the UI from 9 to X isn't much of a factor to me -- yeah, it takes me a millisecond or two longer to do something in X, but I'm good at navigating the UI of both systems -- I'm not one of those people who goes searching through the menus and what-not looking for what I need -- I know where the stuff is I need and I go directly to it. I wouldn't say my productivity has decreased in X from 9, and I wouldn't say it takes any longer either. I'm sticking with X from here on out. No more 9 for me. If you stick with outdated technologies, you're bound to be MORE disappointed than if you jump on the train and follow the new technologies, even if that means using it on an underpowered system. The speed of OS X is catching up now, and I think that there will be no need to go back to 9 once Panther is released.