Sparks, that's a funny question ... any computer is obsolete the moment it leaves the store.
Seriously though, it really depends on what you are doing with your system. To put it another way, a P3 500 MHz system is more than fast enough to do everything except video encoding and very serious game playing. So, if you weren't a gamer, it wouldn't be obsolete to you. If you were a gamer, it was obsolete 2 years ago.
I think a P4 3.06 (533 MHz FSB) should be an excellent system for a few years at least. That said, my opinion is to wait for the 800 MHz FSB P4s to become available. (I believe the release time frame is early May.) At that point I would get a new motherboard that supports 800 MHz FSB P4s. The release of 800 MHz FSB P4s will cause the pricing of current P4s to drop significantly. The 2.53 B is currently about the best performance/price on the Intel side of the fence. In mid-May the 2.8 GHz may have fallen to the same price. If you do this you will be able to upgrade the system with 800 MHz FSB P4s for as long as they are supported. You will also be able to take advantage of the newer features like AGP 8x support, Serial ATA, and other features that have been added to the new chipsets. (The i845PE lacks some features more "modern" chipsets like the Granite Bay and nForce 2 [for AMD] have.)
As for SiS chipsets, to be frank I've never cared much for them. I have had very bad experiences with some of them in the past. That said, the new P4 SiS chipset is among the fastest chipsets available for the current batch of P4s and is very inexpensive. On the downside, I don't believe it will be able to support 800 MHz FSB P4s either. When I do an Intel P4 based system, it always goes on an Intel chipset. With the exception of the new Granite Bay (E7205) chipset, that's always yielded good results. (Intel has now officially admitted that there is an AGP 3.0 [8x] bug in this chipset and it will not be fixed, pending the release of newer chipsets.)
