No need for dual-core for a webcam. Memory upgrade to 1GB, perhaps 2GB as memory is so cheap at the moment. No need for more with XP. click here
Not much you can do regarding CPU on its own, sourcing a replacement and fitting probably not worth the effort for what may be an unnoticeable performance improvement - if you elect to change the whole board, that's a different story - get the whole bundle in one; click here
If you decide to go for a motherboard upgrade you'll probably need a more powerful power supply too, and if your graphics card is an AGP model it will need changing for a newer PCI-E version. Dell's PSUs are generally pretty high quality but the current one may not be powerful enough for the new kit.