Don't worry about HD space, 150GB is plenty for those two games.
Now, to check your system specs (the stuff you need to know about your computer), assuming your using Win XP:
Start>Control panel>Performance and Maintenance>System
When you click system, it should open up a new dialogue box that has a paragraph down the bottom starting with
Computer:
Below this, it will come up with basic PC details (CPU, CPU Speed, then RAM) Once you know that, it's time to check your graphics card.
For this part, click on system in the control panel, and where the tabs are at the top of the dialogue box, click hardware. Then, click device manager. This will open another dialogue box. A bunch of crap will show up, and you'll just need to expand the section where it says "Display Adapters" Expanded, this section will tell you your graphics card. Once youve checked, reference them with the recommended sys requirements for each game.
Recommended for Oblivion:
* 3 Ghz Intel Pentium 4 or equivalent processor
* 1 GB System RAM
* ATI X800 series, NVIDIA GeForce 6800 series, or higher video card
If it doesn't meet these, you'll either be playing at low low low low graphics, or not playing at all. I can't imagine spore needing a better pc than oblivion. Hope this helped!