Upgrading Pre-Built Computer

Basically if you look at the pricing of @Lambdaben 's build and add up the cost of the GPU, PSU and chassis, you get $560. The motherboard, memory and processor have a promotional bundle for $350, which puts the total at $910 before taxes. Should fit your $1000 budget.

Fitting in a 40 series GPU or 7900/6900 will be a bit difficult just based on their pricing alone. What resolution/refresh rate do you intend to game at? The 6700 XT is a solid 1080p card (60 fps in most AAA RPGs, 120+ in competitive shooters). To fit a higher GPU within that $1000 budget, you'll need to compromise on the CPU and board quite a bit, which I would personally advise against.

If you do not have a decent gaming monitor, I'd try to factor that into the budget. You'll have a better experience using a 144hz 1080p VRR panel and 6700XT over using an RTX 4070 with a 1080p 60hz panel.

If you don't mind going with an AMD platform, you can save an extra $50 using this bundle: https://www.microcenter.com/product/5006542/amd-ryzen-5-5600x3d,-asus-tuf-gaming-b550-plus-wifi-ii-ddr4,-g-skill-ripjaws-v-16gb-ddr4-3200-kit,-computer-build-bundle. If you already have a gaming monitor, it might allow you slightly more budget to put towards a slightly better GPU, like an RX 6750XT or 4060 Ti.

They can, however I believe there is a service charge for the labor on that. You can find the pricing here: https://www.microcenter.com/site/service/instore-service-complete-build.aspx. Service starts at $149.99 and goes up from there depending on the complexity of the build.