A 2008 Mac that came with a 2600XT would have to be the old aluminum (aloomineeum!) Mac Pro, most likely the Early 2008 model, so we’re talking Xeon Quard Core processors and a spindle drive along with that 2600XT.
Although a pretty hefty machine for its day, it’s getting a bit long in the tooth. Still plenty of CPU power for most tasks, but light on GPU power. Unfortunately, there’s just not a good cost effective path to beefing that machine up to be a great UE4 specs, I’m sorry to say.
The best graphic card officially supported in that machine is the ATI Radeon 5870. For its day, a powerhouse, and still capable of running UE4 (my secondary machine is a 2010 Mac Pro with this card and I’ve run UE4 on it without too much frustration). You can probably pick up one of these for $100 - $200 on eBay, and it’ll work. It won’t be superb, but it’ll be quite usable. All-in-all, this is probably your best option if you’re not interested in dropping a lot of money.
There are third party unofficial mods to make more recent graphic cards work in aluminum Mac Pros. There are people on eBay who mod cards like the Nvidia GTX 980 to work in the Mac, but you’re in the realm of unsupported hardware, and you’re going to drop a fair bit of money to get an aftermarket modified GPU, especially a hefty recent one. I’m not even sure if the graphics bus on that machine is fast enough to really take advantage of newer GPU hardware. I would research very thoroughly before making such a move. You can probably update your Mac to be good for UE4, but I’m not sure it’s going to be very cost effective.
Unfortunately, the situation with Apple GPUs right now isn’t great. With the exception of the highest-end models of the MacBook Pro, the Retina iMac, and the really expensive garbage can Mac Pro, most Macs shipping today just don’t have good GPUs, and even the discrete GPU in the MacBook Pros is at the low-end of acceptable for doing UE4 work. Apple has been focusing on energy efficiency more than raw power lately and gaming still just doesn’t seem to be a big concern for them. Even the Mac Pro, which has two powerful GPUs in it, isn’t as good as comparable PC hardware both because the FirePros are workstation class GPUs and due to the fact that Mac OS lacks the CrossFire support that would allow UE4 to easily leverage both GPUs.
I’m a Mac guy through and through, but if I were in the market for a new machine primarily for doing UE4 work right now, I’d be looking at Windows boxen. 
There’s always the hackintosh route, but IMO, that’s just inviting trouble and headache.
