You say you have a FX5700 with 20MB, most new cards are now 128-256MB.
I have used a FX5200 128MB in a very similar spec machine but my power unit is 350watts. So it may be a power issue. Nowadays most systems come with either 350watts or 400watt power units to be able to support these new power hungry graphic cards etc. There may be some info on the graphic card by way of minimun spec machine required in a manual you may have got with the card. have a check in there before you do anything like go out and buy a new power unit. Also im sure you'll get other suggestions regarding this post.