In games I felt 4GB too tight (Battlefield plus Win7 and services using 4.25 GB) and OP has only 2 GB old slow RAM, tje option I found is 235 GBP and it blow to air present OP PC with ease including SSD.
I can not comment on the store, it took me 5 minutes to find this offer, I need more time to search GB stores. This was basically just an example of not so old refurbished Dell versus upgrading OP's grandpa. All come here to money, SSD is good edition, no argument, but it would not increase performance during gameplay as expected, processor and video card will. However, Pentium D is very old and slow by present standards, upgrade to 960 would not increase performance dramatically in game, just a bit. If it was PC with Core 2 Duo or something similar - it would be different story.
It is late here, so I will start searching around for more UK retailers in following days, aiming to around 330 before VAT correct? Or you can go more? 450 GBP will buy you brand new cheapest Dell!