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


Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24&