Video Card Upgrade Queston

I have a desktop PC that I used for FSX and Open Rails (MS Train Sim) but which has been unused for sometime now.

The full spec is as follow:

ASUS A8N-SLI with AMD Athlon 64 x 2 cpu 3800+

4Mb of RAM DDR PC 3200

Radeon X700 PCI(E) with 128mb RAM

Philips PHL1500 15" monitor with 1014x768 max

I am thinking about rekindling my interest in FSX but would like to get a bigger monitor (22 inch) as the Philips isn't the best of monitors.

I am aware that the cpu isn't the best for FSX but I am not in the market for spending a lot of money on upgrading or replacing my system. 

I was wondering whether replacing the video card would allow me to use a bigger monitor and give some improvement in the display at a low cost.

In order to keep costs down I was thinking of a used video card something like a GT730(64 bit version) could be a low cost option if it's compatible with my system. Also read that a GTX750ti. could be OK but think it will be too costly. If there are others I should consider I would welcome comments.

As I said I know my cpu isn't the best for FSX so if at the end of the day the best advice is not to bother trying to upgrade the video card then I will go with that.

I am incidentally based in the UK so would welcome any advice on UK sources only please.

Thanks