I'd say upgrade your PC first and splurge on a tablet second. The better your system is, the more you'll appreciate having a tablet at all when you're using it. Pressure sensitivity is great at high levels, but a lot of applications can help you out through the opacity and flow of brushes - so you won't feel any significant loss there. I think my ArtZ II has 512 levels of sensitivity and I'm doing fine. The Graphire4 Classic XL already has a transparent "frame" overlay, so Peacer's last suggestion is taken care of. As for more pens, I definitely second that - I'm still using the original I got years ago (Wacom stuff lasts!), but there was a scary time when it nearly fell apart and I would've died ;P
All in all, go for the first model. Wacom's only fault is their pricing, but you really do get what you pay for.