When it comes to buying a new graphics card these days, you'll want one with 8GB GDDR5 RAM if you want to be playing new games on the highest graphical settings for the next few years. That said, if you're on a budget, a 4GB card will get you by just fine if you don't mind a downgrade.
AMD's Radeon RX 480 has both 4GB and 8GB versions, and obviously, you get higher performance with the 8GB version. Normally you would pay extra for the bonus memory, but Amazon currently has a Sapphire model for £200, which is not only the cheapest this particular model has ever been seen for, this is usually what a 4GB RX 480 goes for. A good amount of power for a budget price isn't half bad. You'll have to wait a few days for it to be in stock, but Amazon should honor the price it's available for right now.
This Sapphire card was up at £230 up until the start of the year, and it's dropped even further since yesterday. The clock speeds are almost identical to the reference specs at 1126MHz base and 1266MHz boost, so it's not the most powerful RX 480 on the market right now, but it is the cheapest.
We've got an in depth review of the RX 480 if you want to make a more informed decision over whether you want this to be your next graphics card. It's a great card which brings high-end performance into the mainstream, but it doesn't set any records while doing so.