Computer help forum: Strange startup problems with Dell Dimension 4700 desktop

This computer is a Dell Dimension 4700 with the Pentium 4 processor and Windows XP. I bought it in 2005 or 2006, and it was used til late 2009 when I started using another computer. I just got it out of storage and I'm having "issues".

So yesterday, when trying to start it up, the screen was blank except for one line of type at the top which was seemingly gibberish, and it said something like "SX-300 blah, blah, 128MB blah blah.

So I Google searched and found someone who kinda had a similar problem, and she said she just removed one of the RAM memory sticks, which forced the computer out of the one line screen and into the set up mode, then she replaced the memory stick, exited set up mode, and the computer went to the main screen and worked normally, so I tried all that and it worked. I was able to use the computer for hours. Then I shut it back off, no problems.

Today, I go back to start the computer up, but this time there is a blank screen with NO type at the top. I clicked the power button again, same thing. However, there is an odd beeping noise that "seems" to be coming from the lower-rear area of the inside of the tower. It beeps 4-6 times and stops beeping. Also, the fan is usually coming on immediately and just keeps running, even though the computer isnt really doing anything. I tried turning the computer on and off several times, but I keep getting the same blank screen, strange beeping, and fan running. BTW: This beeping noise is completely different than any other normal beeping noise the computer makes on normal startup or whenever.

Also, I did try yesterday's method when I was getting only 1 line on the dark screen, where I remove a memory stick, forcing the computer to set up area, which seemed to work at first, but after exiting the setup area, the screen goes blank again, and I'm back to square one with beeping, fan running, blank screen.....

I Googled it, and someone had a similar problem, where a couple people mentioned something about the power supply maybe being bad. But that person didnt have the 2 very different strange startups 2 days in a row like I had.

What do you think, and how do I tell if its the power supply? Is it easy to replace? I found a power supply for sale for a Dell dimension 4700, but in the picture, this unit was long and narrow, whereas my power supply, which is stock, isnt long and narrow at all. Its just about as wide as it is long. It also looks like it isnt going to come out very easily due to its height, unless the fan housing and maybe other things are removed.

Thanks in advance....