Who cares if SB is faster if it won't fit in your budget? $600 - GPU = ~$400. The good SB alone is another $200 or so, not counting mobo. You're likely to spend about $350 for the CPU and mobo, far more then your $173, and nearly chewing up all your remaining budget.
SB is great, but it will take a long time for it to fit into the more budget builds. You could grab one of the dual core budget SB, but I don't remember them doing well.
Edited for spelling.