How do I get my linux partition to overwrite xp partition and take over whole hd?

Unless you have a load of personal data to keep I'd just reinstall and pick 'use the whole hard drive' as an install option.

You can use a USB flash drive to put things on you don't to lose. If you don't own one already, current prices are around £12 for an 8 GB one or about £20 for a 16 GB one.

The new 10.10 version is due within the next week or so, and I'd be inclined to wait for that myself

So that gives you a bit of time to archive data to USB flash drive or DVD('s)

.... and yes I have a native installer for Quake 3 too, it runs great in Linux and slightly faster than the Windows version. Or at least it used to, because I haven't run it for years.

If I get time I'll try it in the 10.10 RC version I installed recently

The absolute simplest way (and most likely to succeed) to do this is to backup your Ubuntu home partition (including all the hidden files), re-install Ubuntu telling the installer to take the whole drive and wipe everything, then restore your home partition.

This approach will get round the whole issue of where GRUB (your multi-boot program) is installed, which depends on many factors such as which OS you installed first, phase of the moon, etc. You see, if you simply wipe the XP partition, you might also wipe GRUB and make the machine unbootable, which you can fix with a Live CD, but that just adds complication.

I've just been through this exercise with my Ubuntu laptop, and it's easier and quicker than it sounds.

You can use Parted Magic for that for example. Simply remove the partition on which Windows XP is installed and then extend your partition on which Kubuntu is installed.

What I would do, and have done in the past, is reformat the XP partition, AS ROOT, copy or move the entire /home directory over there, edit /etc/fstab to mount it as /home when it reboots and free up the home space. Much easier.