HouseOfBamboo - regarding a dedicated graphics card - those are common enough as additions for desktops (one of my PCs has 2 cards, which can be linked for faster handling but I'm using them to drive two monitors). Games players often go for extreme amounts of speed, and sometimes pay 200-300 pounds just for the graphics card (though other cards may start in the upper 20s).
However, it is far less common for any laptop to have an option to add cards as space is at a premium, though there can be differences in how graphics are handled (different chip sets on the laptop motherboard) and a laptop specialist shop would be worth asking if you wish to compare graphics features and find something for your needs. Otherwise, go for 3 to 4 GB of RAM and ensure you only run your graphics software when you have shut down other applications, to get best speed.
Many machines 'share' some of the memory between processing (running applications) and the video display, and the more memory needed for graphics (size of screen[s], user defined settings of pixels for width + depth) the lower the free memory for running applications and therefore the slower it may work (and a minority of software may not run at all, though that's mostly gone away with Windows XP and later, compared with Windows 98 and earlier).
As for the Mac, you can probably pick up a s/h model on Ebay which could be a better option to use as a standby, as some features might be poorer on the PC version of the same software, or awkward to use with different control keys - it has been all too often my experience that some old software won't run well on new hardware (or can be incompatible with a newer operating system, and perhaps have no support if the software firm has been bought/ closed down). In the case of Windows 7, for example, many of the newer PCs use 64 bits for the processor (rather than the older 32 bit processors) and software firms will be aiming at the 'new' market with more interest than for supporting old hardware/ customers.
It was the same for users of Mac OS 9 not long after OSX was available... while some drivers for OSX were unavailable a friend had to run some software in OS 9, and other software under OSX. What a pain! Nowadays I suspect few firms would be interested in supporting a user of OS 9 (or old versions of OSX prior to say 10.2.4 or 10.3.x)