well it will be a bit pointless spending money if its for the sake of it as generally your cpu and memory will probably be limiting your gaming more than your graphics card as your spec isnt very high like you said.

but for £50 any of these 3

http://www.microdirect.co.uk/(16425)Sapphire-ATi-Radeon-X1550-256MB-550MHz-400MHz.aspx

http://www.microdirect.co.uk/(17779)Zotac-nFire-7300GT-256MB-AGP-Video-Graphics-Card.aspx

http://www.microdirect.co.uk/(16429)Sapphire-ATi-Radeon-X1550-512MB-550MHz-400MHz.aspx

just copy paste