Kate,
The video card specified is not sufficient. There are lots of complicated explanations, but to keep it really simple if you look
here at Tom's Hardware, they arrange video cards in approximate performance tiers. For Virtual Reality (Occulus Rift or HTC Vive), you need a video card in the top four tiers (the higher the better). Tier 4 minimum, Tier 3 recommended. The AMD cards are generally a bit cheaper.
Desktop GPU Performance Hierarchy Table
Nvidia GeForce / AMD Radeon
Iier 1 GTX Titan Z / R9 295X2
Iier 2 Titan X, 980 Ti / HD 7990, R9 Fury X
Iier 3 GTX 980, 690, Titan Black / R9 Fury. Fury Nano
Iier 4 GTX 780, 780 Ti, 970, Titan / R9 290, 290X, 390X, 390
A number of online retailers offer finance, so shop around. Don't just buy from somewhere local. For instance
this PC from Scan.co.uk would be more than up to the task (being at the top end) and they offer finance.
CCL Online sell a VR range with 3 year warranty.
Ebuyer.com sells a number of gaming PC's starting from £850.
I am sure other forum users here can point you to many other sources, so you have plenty of choice.
The minimum specification for Oculus rift is
Video Card NVIDIA GTX 970 / AMD R9 290 equivalent or greater
CPU Intel i5-4590 equivalent or greater
Memory 8GB+ RAM
Video Output Compatible HDMI 1.3 video output
USB Ports 3x USB 3.0 ports plus 1x USB 2.0 port
OS Windows 7 SP1 64 bit or newer
and for Vive is
GPU: NVIDIA GeForce® GTX 970 / AMD Radeon™ R9 290 equivalent or greater
CPU: Intel i5-4590 / AMD FX 8350 equivalent or greater
RAM: 4GB+
Video Output: HDMI 1.4 or DisplayPort 1.2 or newer
USB Port: 1x USB 2.0 or greater port
Operating System: Windows 7 SP1 or newer
So the shopping list is a Tier 4 or better video card (you have the list above), 8GB of memory (4GB is pretty small these days) and for the Rift, 3 USB 3.0 ports. USB 3.0 is newer and the ports are coloured blue, so don't get caught out by not having these. Some PCs may only come with two!
The processor (whether Core i5 or i7 - four brains or eight) should be 4590 as a speed rating (this is an equivalent speed rating, not what's know as the "clock speed").
I hope this information is helpful.