The key question for me is, do you really need to upgrade at all let alone immediately? Having read a magazine article on this, it revealed that it is more than possible to enhance Windows XP to offer some of the features in the upgrade - for free.
For example, the visual appearance of the desktop. Try downloading Windows Blinds. Then you can use it to customize the theme of the windows and desktop to match Windows Vista. In addition, Yahoo Widgets will enable you to create Vista's desktop gadgets - also for free.
(above) Windows Vista gadgets
(above) Yahoo Widgets gives XP gadgets
Another tip worth noting is that you can adjust your text to cleartype in XP to match the style of Vista. This is done by right-clicking desktop and then appearance tab, effects box and tick clear type box. The smooth Windows Vista text will now be on XP.
Furthermore, Windows Vista proudly boasts the latest version of Internet Explorer. But remember, this isn't just available to Vista. IE 7 is also available for XP. Download it from the MS website and it can be yours for nothing.
Vista comes with a new image editing (Photo Gallery) tool as a replacement for XP's slideshows. However, you can obtain a very similar image editing tool for nothing. Download the excellent Picasa freeware from Google (http://picasa.google.com/) and you will have virtually the same thing on Windows XP.
These are some of the things I have tried. There are also things you can do with XP to adopt the security features offered by Vista amongst other enhancements. However, this is already dragging on and I believe I have made my point.
If you plan on upgrading, you might want to reconsider if using XP. At least for the time being while the price is high.