Legacy to UEFI? GPU Compatibility?

[zGoDLiiKe]

First of all any help is much appreciated. I have never built or done major upgrades to a PC before.

 

TLDR: Are new GPUs compatible with Legacy bios? What are my options?

 

I have a Dell XPS 8700 that I would like to breathe some life into. It has an i7 4790 CPU, 24 gigs of RAM, SSD, couple HDDs, and a really outdated and basically useless GPU. I am hoping to upgrade the GPU and also the power supply. The long term goal is to build a new PC at some point but I figure the GPU and PSU are a good start that can be swapped to a new build. 

 

After doing a bunch of research about size requirements (the case is constrained, MOBO obstacles, etc.) and measuring I am hoping to get a 5700 xt or similar. Then I read about BIOS compatibility and people said new cards (especially 3rd party AMD cards) often don't work with Legacy bios. Is this true? Anyway around this?

 

I went into the BIOS settings and the computer is indeed in Legacy BIOS, the drives are MBR etc. so if there is an issue can I use an MBR to GPT converter on boot drive (or all drives?) and then boot in UEFI mode?

 

I really appreciate any insight that you may have!