What is your current graphics card? If you have a GeForce2-level card, I would highly suggest purchasing an ECS K7S5A motherboard with an AthlonXP 1600+ processor. Both are available from www.newegg.com
for less than $60 each. If you have a GF2, it will scale well with that new setup until you can afford a more powerful graphics card.
A Ti4200 won't do all that well on an 850 MHz processor. Going the new motherboard CPU route would give you a speed boost with your present graphics card and the K7S5A will still allow you to use your current single data rate memory, which will save you more money in the short term.