It of course depends on what you are doing. If you are surfing the web, reading email, and maybe playing a few games, then NO, you won't see a difference. You probably shouldn't bother upgrading, until amd's next version(thoroughbred(sp?)) comes out, and you can get it for $140 - $150ish retail.
I basically just built a Athlon XP 1600+, which is overclocked to 1500.5 Mhz. It has 512MB of DDR ram, and a 7200RPM harddrive. It also uses the new kt266a chipset. Before I had a PIII 500, with 512MB PC133(running at 100) RAM, and a slow harddrive.
I can tell a HUGE difference in benchmark tests. Can I tell a huge difference in reading email and browsing the web, NO.
I will be doing some software development and CAD, so I will see some improvements there.
The thing is, your Tbird 1.1, would have smoked my PIII 500. I've ran benchmark tests, and if you do cpu ones, then my XP 1600+ running at 1400Ghz, doesn't do any better than a Tbird at 1400Ghz. But I do smoke it in memory tests and multimedia.
So it all depends, if you are doing 3d modeling and rendering, then yeah you will see a small difference. If not I wouldn't bother for awhile. I think you should probably wait for their next line to come out. Hopefully they will have, 1.7, 1.8, 1.9, and 2.0 ghz models. Then maybe you can pick up a 1.8 for relatively cheap. Who knows when this will be, but you may just want to wait, you already have a screaming machine.
Now if you MUST, I currently have a Gigabyte GA-7VTXH board which has the KT266A chipset. Along with that I bought a Athlon XP 1600+ processor, Antec Workstation Tower Case(comes with 2 fans), Maxtor 40GB 7200RPM ATA133 Harddrive, Pionneer 16x DVD Rom, 512(256 x 2) PC2400 DDR RAM(OCZ Technology), ATI Radeon 7500 Video Card(OEM), Microsoft Elite Keyboard, and a Logitech 2 button w/wheel optical mouse.
Basically, I reused my burner and monitor. With all of that in mind, I did it for under $800 with shipping included. I would be more than happy to post any Sisoft Sandra benchmark tests, if you want to compare them to your system.
James