If you don't want to try and flash one yourself (you have to have a PC hanging around and XP won't like you changing graphics cards... it'll make you re-register) so it's best to have a '98 machine hanging around or something), then there's a guy on eBay selling flashed 9800 Pros for around $99, which is a vast improvement over some of the people previously selling them for $150-200 (talk about a money maker). I bought one and it works fine.
Your biggest problem with Leopard IS going to be the graphics card. I've got a 1.8GHz 7448 G4 upgrade with a 9800 Pro and and Tiger is STILL noticeably faster than Leopard, not mention many older games won't like Leopard and you lose the OS9 Classic mode (for even older games). I've kept Leopard up and it's quite usable now, but I still run Tiger on a daily basis. I have exactly ONE program that only runs in Leopard and that is Handbrake (and I generally use my PC for that anyway since it's way faster with two 2.8GHz Athlons in it). The need for Leopard is way overrated, IMO. About the only thing I like on it better than Tiger is Spaces and I hardly ever use it (even on my Linux machine which has Compiz and rotatable cube effect, I don't actually USE it much except to show off the 3D effects and Leopard doesn't even do anything cool like that with it).
So until iTunes no longer gets updates for Leopard, I'll probably continu to run Tiger on this machine for daily use. I mostly use it for browsing/shopping (secure stuff) and to drive my whole house audio system (using multiple AppleTV units and iTunes8). But then, that's mostly what I use computes in general for (browsing, shopping and audio/video entertainment), which means my PC is mostly for gaming now and game development.
