Andy,
Are you using AGP or PCI-E? Here are a couple of choices from me in the not too expensive ranges:
AGP: Sapphire X1300 256MB - £64.57 inc VAT
PCI-E: Asus ATI Radeon EAX-X1600 Pro-TD 256MB - £71.62 inc VAT
As you can see, the PCI card is a bit more expensive, that is because its a far better card. The equivelant PCI version of that AGP card comes in at about £42 so you may also want to consider changing over to PCI-E motherboard when you get some spare cash. (Of course that is if you don't already have one!)
I prefer the ATI cards myself although for FS2004 there is not a lot to choose in the performance difference between manufacturers. By the way, both of these prices were from
www.overclockers.co.uk a cheap mail order component supplier. Have a look at "This Weeks Offers" as they often have some great deals on GPU's.
With regards to the cards above, it is my opinion that you will not get a good enough performance from either of the above cards to run FSX satisfactorily at high settings. I had an old X850 and although it is a great card, it struggles with FS2004 at full detail. (This is a similar card to the X1600)
Hope my opinions help.