You could have picked something better then PC World! I’ve never liked their products and the prices are punching above the weight considering you can get better quality machines elsewhere with higher specifications then the ones being shown here.
Personally there isn’t a huge difference between the 3, considering they’re £30 apart. I’ve run MTA smooth with 512MB (DDR2) in the past so with each containing 3GB (DDR3) of RAM you certainly wouldn’t stumble across any problems with performance. The £249 comes with a fair dual core, whilst the integrated graphics by Radeon will provide stable performance to run SA.