for corel and photoshop use I think you will be better with upgrading the CPU rather than the GPU, however, both would be good too.
I would recommend an Nvidia card because CUDA acceleration is implemented in newer photoshop versions. I think that a GTX 260 or 280 would be a great value purchase. You can find them on Ebay for really cheap compared to the power they provide.
For the CPU go for a quad core core 2 either a Q6600 or better. I am sure photoshop uses multicore, not sure about corel, but any boost in clock speed will help.
I would say you could get those upgrades for $100 (gtx 280) and $90 for the CPU from a quick glance at buy it nows on ebay...