I'm a soloist too. It is hard to recommend what to get without knowing your future plans. Right now, you can get alot of used equipment for next to nothing...they might not run on Windows XP because of driver issues. You might need an ISA slot for an older card and P4 motherboards do not offer ISA slots. The sound quality will be the same. Anything you get will be better than the cassette though. I had a Tascam 238s 8 track cassette which gave me a clean sound but I still had to bounce which degraded everything in the end. For a basic setup I would recommend a PIII (anything but faster the better)/128+megs of ram/8gig+ hardrive...you could use a PII and pick up a loaded 450mhz for under $150. Get a good Audio card (not a multimedia card). I still use my Card D+ (ISA). It was $1200 with the I/O daughtercard, used by major labels, and you can now find them for as low as $70. GINA/DARLA may not have the same quality AD/DA convertors but they are popular too. Other people will recommend good cards...just make sure it is an AUDIO CARD and not a (soundblaster type) MULTIMEDIA CARD You can get a CDRW for under $60 and the blank CDs/CDRWs are free from CircuitCity type stores (watch for rebates...I also had over 2000 megs of 133mhz ram sticks I got free from these stores)...Cakewalk Pro Audio 6.0 or higher(I've seen CPA 9.0 for $50...you might get someone to give you an older version)...Mackie 1202 (around $125-150)or (1202vlz much better)...
This will give you a min. of 16 audio tracks (up to UNLIMITED)and unlimited MIDI tracks. I had a Pentium I 150mhz that did all of the above...it just took forever to do any master editing of 24 audio tracks with Sound Forge.
If you are looking to the future than get a Pentium 4 (they didn't get many bugs out until 1.7) with 256+RDRAM (that is what the P4 is optimized for)and 60gig+ hard drive. I would do research to make sure any addons had drivers that would work with XP (or do a dual boot with XP and 98se). Make sure the motherboard supports addons. Get a great soundcard and then expand as you can. It will cost alot more in the long run, the sound won't be any better BUT there may be a piece of software that you have to have in 2 years and a PIII won't support it.
You might want to check out www.harmony-central.com
classifieds. I just sold a 233mmx loaded with tons of software (CAkewalk, Sound Forge, Cubase, etc.) and a Card D+ for $200 just to give you an idea of what to look for.