Ummm... your question may seem obvious to you, but it can mean lots of things.
What sort of 'speed' increase are you hoping for? If you're specifically looking for an increase in video, for example, assigning more graphics memory can help. If it's taking a long time for programs to load, you can assign a separate disk (especially, say, an SSD disk) for the virtual machine's exclusive use, allowing the VM to directly access the disk, rather than having to use the host's IO filesystem. Finally, there's lots of tweaks you can apply to XP itself, like turning off redundant services that are already taken care of by the host i.e. disabling the firewall, and various network systems. And, most importantly, install the virtualbox guest additions, as that has a huge impact on system responsiveness.