Well........i would also suggest getting a whole new pc!
if you want a pc that would last you a looooong time with minimal amount of upgrades here is my suggestion:

a new motherboard that supports intel dual core, core 2 duo, quad core cpu's. If you dont want to spend too much on a cpu right now go for a dual core like 3.4Ghz. You can always upgrade it to a more expensive one later if you buy a good motherboard. Now, RAM, 1GB is....ok......you must remember one thing, if you have only 1GB in your machine and you are using poser windows is probably using half that just being on, and rendering with hight resolution textures will eat the rest up like cake and it will crash!! I would suggest getting at least 2GB. Poser on a good rendering day will use at least 1GB while rendering, if you have memory to spare it wont crash! and your rendering time will go down. Memory speed with a new motherboard should be DDR2 800, i suggest 2x 1GB chips. harddrive, if you buy the new motherboard get a new SATA2 drive, one with a 16MB cache on it. these drives are nice and fast and windows will allocate data on it much faster that other drives. size is up to you! graphics card, well this is up for debate! do you play games? do you only use poser? any 7 series card should do nicely, the GT based cards are faster but they cost more. the only difference with graphics card would be hardware shading in poser, your preview would be better if its enabled. if you also play games get a card with lots of memory and one with 256bit memory on it.

a new multicore support motherboard will take you far.
i upgraded my pc a month ago and poser is flying now! the pc i got is as follows:
3.4GHz Dual core CPU
4GB DDR2 800 Memory
500GB SATA2 harddrive
Geforce 7950gt Graphics card.
Windows Vista Ultimate.

i would also suggest a 64-bit operating system but i dont know if poser supports that.
hope this helps you!!