Welcome to the forums Tom.
You don't want that system. It is a rather old server system. In fact the way the seller tries to say it is a "great gaming machine" I wouldn't buy anything from that seller because he/she is very much full of it and I wouldn't trust him/her farther than I can throw my house.
Sure, eight years ago that would have been a great gaming platform, but right now you could buy a standard off the shelf cheap system for less and it would blow it away.
Xeon 2.8Ghz processor = old single core processor. We are currently on six core CPU's. Dual core is the minimum you want, with gaming quad core is highly recommended, six core if you are building new.
SCISI = big bucks
AGP = three generations old video card slot
6200 video card = can't get much lower than that. From the Sellers description -Maybe, just maybe it would actually be able to run The Sims 3, but only if you like slide shows at the lowest possible resolution.With dedicated video 512MB RAM with Pixel Shader 3.0 support
Capable of playing the most popular games, including The Sims 3 and World of Warcraft
1GB DDR-266 = I have 1GB ram on my video card alone. 2GB system memory is standard for an office computer these days. Gaming should be 4GB. Single channel DDR-266??, current mainstream memory is DDR3-1600 dual or triple channel! And even faster speeds for those with deep pockets. What a difference!
What is your budget? We can certainly help you find a much much better system than this old dinosaur.
You know what I would pay for that system? About $25 (about £12 ??). Maybe $50 if it actually had the second Xeon in it. That's about all it is worth. No joking in any way, I have given away better systems.