Most of the following relates to using XP Pro as host. As soon as you feel comfortable using Ubuntu or some other Linux as host do it since you will see a performance increase in the VM.
On VirtualBox, its like all other VMs from other manufacturers, if you can afford to throw more RAM at it, go ahead and do it since VMs are like JAVA, real RAM hogs. RAM is incredibly cheap right now.
The other area they like to gobble, is hard drive space. Usually 8 to 10 GBs at a time. A large hard drive is an advantage here. Intuitively putting the VM "virtual hard drives" on a drive other than the boot/system drive for the XP host should help performance.
Unfortunately by default VirtualBox will place the VM hard drive files in the Document and Settings User profile folder. Ugh. Be sure to put them some place else to prevent massive profile folder bloat.
When it comes to operating systems, especially Linux distros, trying as many as you like, yes you can. Create a new VM for each distro. Just remember that a running VM will grab RAM space. The more RAM you can give it totally uncontested, the faster the VM will run. So to speed up VMs, limit the number running at a time and give them as much room as is practical. That means if you aren't using it, suspend it or shutdown that specific VM.
Beware that the hard drive you have all of this running on is in good shape. What can't run or isn't running in RAM, is being swapped out to the hard drive. So drive swapping tends to increase rather remarkably. A high speed drive helps but its still way freaking slower than more RAM.
Each of the VMs will have a setting for the amount of RAM to allow the VM to use. That comes out of the total amount of RAM of the host. The host requires RAM space since its actually fairly busy responding to the services demands from the VM so don't totally choke off the host. Setting RAM below 512 MB on XP Pro SP3 system without shutting down a lot of the unneeded XP services is likely to generate a big performance hit to the VM.
When you get comfortable with Ubuntu (or another Linux distro) I find that running VirtualBox on Ubuntu as host, things runs faster than with VirtualBox running on a XP Pro host. Mostly because there isn't so much crap running like in XP Pro. But also because the OS can use the RAM above 3.7 GBytes like XP Pro can't in a 4 GB motherboard. This is an OS limitation based on a hardware issue. Might be different in differing OS or hardware. All my testing has been on Intel chipsets. Not a problem on 64 bit systems. The biggest argument I see for a 64 bit board is a VM.
Also another trick is to shut down as many services as you can in the XP host so that you can to give the VM more room. However do NOT shutdown networking, firewall, SMB, RPC, DNS, DHCP, workstation, secondary logon, error reporting, event viewer, or printer services. If you just stop the services instead of disabling them, when the system is rebooted they come back. There are others as well.
One special service ABSOLUTELY that needs to be turned off temporarily is Windows Update, especially if you have set it to auto-install. You don't want it trying to update while the VM is up and running! Especially since it seems practically ALL XP updates require a reboot.
You can shutdown dotNet related services, indexing service, help & support services, DCOM, Terminal Server like remote desktop and assistance, and anything related to dialup server (its not likely to want to stop) and you're not likely to need Hyperterminal!
In the case of Dialup Server and Telephone related services, if you aren't using a dialup connection, you can disable them but it requires a reboot to make it "stick". Just set them on disabled and then reboot. Its almost impossible to get the telephone services to shutdown from the MMC or Services applet. When the XP system comes up the services won't be running.
In the process window in Task Manager, you can also stop any of the software update processes, like Google, java (juschedule), realplayer etc. If you have a variation of SQL server or its agents or writers running in the XP host, shut them down. If the OS comes back and tells you that the process is vital, believe it and try shutting something else down. Anything with svchost attached to the process name is likely networking related so just leave it running.
If something stops working that you need to have running, just reboot the XP Pro host. As long as you haven't gone into the Services applet and DISABLED the service, it all comes back on the next reboot.
Please note that all of this "advice" is related to Windows XP Pro. I haven't much experience with Windows XP Home. In the areas of running VMs on XP Home, I've had no experience. I've considered XP Home the "runt of the litter" and haven't paid it much heed. My kids run it on their systems but that's about it.
BTW all of this advice works for everyday XP Pro operation or game playing even without a VM running!