I dont think a 2200+ vs a 2.4B is a fair comparision, money wise. A fairer comparision would be 2200+ vs 2.26B. They are about even, depending on how you set your p4 (Rambus, DDR). p4/Rambus will be the fastest. RDram does give you a lot of headroom to overclock, if you know what you are doing. Go to vr-zone.com, the top p4 overclockers use RDram.
For instance, if you overclock a 1.6a to 175FSB ( i've seen a few on this forum, and you use RDram, then you have 175 x 3 x 2 = PC1050 RDram, no where near its limitation).
I wouldnt recommend a Sis645DX board, go for i845G or E or i850/i850e based board.