If* Director MX is not compatible with Vista (seems odd), then you need to decide between that program's value to you versus the value of Vista's new features (which are?). I would have thought that any recent program that's compatible with XP would be compatible with Vista - unless it's a 64bit issue, in which case it won't work with XP x64 either.

Honestly, what value is a 64bit operating system to you? Will you benefit from it? Going by the hardware you list for your new system, you will not benefit from it at all. Indeed, if you have any important peripherals like printers or scanners or graphics that don't have signed drivers for Vista 64bit, then you'll suffer for having it. They won't work - full stop.

I don't see what's silly about buying a second copy of XP. It's a well sorted OS with guaranteed support for years to come, that already works with the programs you want to use. You can buy a full version for less than sixty quid, and your hardware is almost idealised for it.

Some notes on Vista. 

If you buy an upgrade version, I believe you have to install over an existing qualifying installation of Windows. You can't just let it sniff your qualifying setup disk and do a clean install as with previous upgrades. Do you have a qualifying version of Windows? Your current XP version won't count if you're still using it (they'll know...). There is a workaround that turns an upgrade version into a full version, and although Microsoft have taken a very relaxed view on it (so far...), it does nevertheless break the terms of the licence agreement. So I won't link to the workaround procedure here, you'll have to search online yourself.

If you buy an OEM version of Vista, your licence will only cover you for the hardware you first install it on. If you change anything significant, such as your motherboard (or even something much less significant) then your licence expires and you need to buy a new Windows! It remains to be seen just how stringently Microsoft will enforce this, but you need to know that you will be at the mercy of MS and may find yourself without an OS should you change something. OEM Vista comes only in either 32bit or 64bit - not both, nor with an option.

A full retail version of Vista allows a clean install and permits transfer (not duplication) of the licence to new hardware. The full retail version of Vista is very expensive - and literally almost doubly expensive in "ripoff Britain". Upgrade and Full Retail Vista - in Home Basic; Home Premium and Business versions - comes as 32bit in the box with an qualifying option for a 64bit DVD. Only Vista Ultimate (Full and Upgrade - not OEM) comes with both 32bit and 64bit in the box.

Yikes. I'd stick with XP.