That system has gone about as far as you'd want to take it...

Of course a better video card never hurts and you can take it along to your "newer better" system..

Any worthwhile CPU upgrade is gonna require a new main-board and we all know what happens once you start that ball rolling    

I'd get the best v-card you can afford (wait till it's a significant step up).. keep the old card to put back in this system when you build your new "super system"...Then you'll have 2 decent computers for what  ever fun you dare get into (lan gaming, backup etc.)

I wont answer the question on bios now, but if you still cant figure it out ask again.
(to change bios is a slightly risky business)

A 2600 will make the 1600 look sick. It runs on a 333mhz front side bus speed so to take advantage of it, you need a mobo that supports 333mhz fsb and PC2700 DDR. (I recommend the Nforce2 platform, Soltek make several fine examples at budget prices)

Your TI4200 is a good fast video card, and there are few which will outperform it noticeably. But, I found that the TI4200 is now the bottleneck in my system, as the rest of it runs faster, (at last)

The Soltek allowed me to tweak my DDR ram to CAS2.0, set my cpu multiplier from 12.5 to 13, Front side bus speed from 333mhz to 340, and now my system benchmarks faster than a 3000+ with a tiny overclock.

You will like it, everything runs smooth as   

the 2400 is your best option  other than a new system, they are cheap, go for it.  

Or........ get a 2600+ in readiness for a further upgrade, but by the time you upgrade you may want a faster cpu.

The 2600 will run at 266mhz as long as your mobo supports it.

you can even put an athlon XP 2800+ onto the KHA+.

but there's no need for pc 2700-ram.

if you want something for the future,buy a Geforce FX 5600 or 5800, but i strongly recommend an ATI 9700pro.