What do you think would be a good graphics card upgrade?

Sometime in the future I plan to upgrade my computer to keep it up to date. It's for gaming, and I have a Radeon R9 270X 2GB. This is more than enough for gaming like Team Fortress 2 and Rust, but for some reason Minecraft seems to run very slowly sometimes, for example today on a server the chunks wouldn't load, but I don't know if that's a problem with my CPU or the host's internet connection.

Can any graphics card fit into any case? I mean, can a NVIDIA or something work with an AMD build like mine? I've heard NVIDIA is better for Intel, and Asus/Radeon is better for AMD, but I don't know. If you had to give me a better graphics card from AMD and one from NVIDIA, in the price range of £200 -
£250, what should I get?

Added (1). Sorry, I have 8GB but I plan to upgrade to 16GB later, probably before the graphics card. The processor is the FX 8350 8 core 4.0/4.2GHz.

Added (2). I'm very sorry for not supplying specs, it is the Asus M5A97 R2.0 AM3 AMD 970 DDR3 ATX, 600W power, 1TB hard drive (I'm adding an SSD before the GPU upgrade) and I'm going to add more system RAM before the graphics card upgrade.

I would suggest an EVGA Geforce GTX 660 or higher graphics card if you got enough RAM and processing speed.

How much system ram have you got?

i would upgrade system memory to 16gb, before you even think about a card.

It depends on your motherboard on which card is compatible, you really need to supply computer specs when asking. )

Looks like your stuck with ATI cards. Nvidia is not an option as not compatible, also i would consider upgrading your psu. 750w min. Just see how you get on with just a memory upgrade, it will help

Don't spend money on upgrading your GPU nor RAM, both are fine. It's the CPU that is the problem. Minecraft runs mostly CPU based, and your GPU is great. Try finding a real high quality AMD processor (if there are any). Otherwise, I would recommend upgrading to Intel, although you'd also have to buy a new motherboard.

The problem is trying to recommend something now, when you intend to upgrade in the future, as anything we will recommend will be out if date.
Your current graphics card is already is already high end, a further upgrade would mean upgrading to a GTX 760/770 or AMD R9 290X which are serious amounts of money.
Whoever says Nvidia is better for intel and AMD is better for ASUS/Radeon is talking nonesense.

8GB RAM is usually enough for vertically all games, you will get little benefit from upgrading.

Your current processor is again reasonable and the CPU support lists shows this is the fastest processor that it supports, so you would need to upgrade your motherboard and CPU to an intel core i7 which would also mean new RAM

Minecraft has fairly low system requirements, any problem are unlikely to be at your end
HTH
David