I'm in the current process of tricking my 2001 Quicksilver..
I managed to pick up an ATI Radeon 9800 Pro 128mb for around $90 CDN shipped. And while the 733mhz cpu is a bottleneck, it gave noticeable improvement on overall image quality of the OS, especially with things like scrolling, Expose, Menus, and minimizing/bringing up windows from the Dock.
OWC sells some daughter card upgrades, but they aren't cheap. A dual 1.6ghz card is about $400 USD.. eBay typically runs on-par or more for similar cards. Mine came with a stick of 512mb RAM and it cost me around $70 CDN to put another gig in, which was quite difficult to find (the computer is almost a decade old).
Given the slow bus speed, and the ram bus speed (both 133mhz, yours may vary), the old G4's can't push data like something newer (possibly a G5 or early Intel), could put out.
I'm a gear sl*t, especially for legacy hardware.
While my G4 is still a usable system, and can run Ableton
alright, I've decided to purchase a used G4 MDD with dual 1.25ghz cpus, which are on a 167mhz bus (not much faster, but it's noticeable), and uses DDR333 ram, which also helps quite a bit. I'm using mine for one specific role, which is running a sound studio, and Ableton takes advantage of dual cpu's, so the G4 MDD would be a decent system for my needs until I get the cash for a MBP, as my Dell Studio just ran my VST's like garbage (even with a core2duo and 4gb DDR2).
I'm technically not paying for the MDD, as I'm selling my current QuickSilver for the same price as I'm paying for the new one. Macs like these go for a lot more than where I live compared to most other places (like the USA), so I can get away with it.
If you're just using it for basic computing tasks, it should be fine running Tiger.
As for gaming, a lot of the newer, better games are made for Intel-only Macs. There are plenty of older games like Call of Duty, StarCraft/WarCraft 3, Dungeon Siege, and Diablo II that can run decently, provided you have a 64mb+ video card and a cpu of around 1ghz.
My 733mhz barely plays CoD, SC/WC3 run nicely, as does Dungeon Siege and Diablo. Halo will also run on anything 800mhz+ with 32mb vRAM.
To be honest, you're better off buying a G5 at the least, or move into older-gen Intels.
You have to keep in mind that a lot of software developers do not make Universal binaries anymore, which prevents the G5 and previous from running anything. And believe me, it's getting harder and harder to find PPC binaries on the Net (prime example, Transmission, I had to download v1.54 to be able to run on my QW).
Not meant to flak, just some food for thought.
Also, posting specs of your current machine may give some insight to the rest of us here on some cheap upgrades that might be worthwhile!