I currently have 10.3.183.86 installed.
Good news, bad news....
First, the bad.
Since you're running 10.5.8, your Flash Player is as up to date as it will ever get. 10.3 is the "end of the road" for PowerPC or Universal Binary (PPC/Intel) versions of Flash Player.
Now, the good.
Because you have an Intel Mac AND it's a Core2Duo (64 bit - CoreDuo's were 32 bit), you can upgrade to OS 10.6 (Snow Leopard) for a paltry $20 through Apple. They ship anywhere for free too.
Once you've upgraded to 10.6, you'll need to install Apple's 10.6.8 combo update and from there, you'll find a whole world of software that's unusable with 10.5, including the latest Flash Player.
About the only thing I'd recommend would be to check OWC.com (OtherWorldComputing) or Crucial.com and purchase a 4Gb RAM pack for your Mac. Snow Leopard will run with 2Gb, but it'll fly with 4.