In my opinion, here are the most important factors in increasing Photoshop speed:
1) RAM
2) HD Space
3) CPU Speed
The ATI Radeon 9200 video card you have should be adequate - don't bother buying a new video card.
About 512MB-1GB of RAM is usually what I consider to be a "minimum" for PS.
If you think you'll be working extensively with large files and layers, consider 2-3 GB instead. PS has a RAM limit of 2GB - so if you purchase 2GB, your OS and background apps will use 300-600MB of that, leaving PS with around 1.5GB. If you purchase 2.5-3GB of RAM, PS will run at its full 2GB limit, leaving the remaining 512MB-1GB for OS and background apps. When considering different RAM types: SDRAM is minimum, DDR is better, Dual-Channel DDR is best.
Empty, contiguous, dedicated HD space is also essential for PS. When working with a 100mb layered file, it isn't unusual for PS to consume 500mb or more worth of "scratch disk". When planning your computer purchase, make sure you have at least ONE empty hard disk which will be dedicated for PS "scratch disk" use only.
PS has a 2GB file size limit on scratch disks. This means PS will only use the first 2GB of each scratch disk volume. To circumvent this limit, partition your empty "scratch disk" hard drive into four 2GB partitions. Set up PS preferences to use each of these four logical 2GB partitions as a different scratch disk.
If properly maxxed out, your PS rig should have 3GB of RAM (PS will run at its full 2GB RAM limit), and four 2GB scratch disk partitions.
In my opinion, CPU speed is the least-important of the 3 factors: For CPU speed - obviously faster is better. PS supposedly has some optimizations that allow it to run faster on Intel processors, but avoid buying into an older Intel-based system which uses costly RDRAM. Stick to inexpensive DDR instead. If you max out on the first 2 factors (RAM & HD Space), your CPU speed will be less important to getting maximum Photoshop speed.
If money is an issue, why bother with Dell? Just do your homework and ask around here on Anandtech for advice. It's easy and inexpensive to build your own system for a fraction of the cost of a similarly-equipped Dell machine. There are now over 100,000 members here who are happy to offer advice for free.
PM me if you have questions.
Hope this helps - Good luck!