Can't point you towards a specific model of fan or fans without knowing your specific Dell model.. BUT luckily there are standard sizes for cooling fans - quite a few sizes - but since we're talking about a 6 yr old Dell, well that narrows it down a little bit. Here's what you should do first: take a plain old ruler that measures in millimeters/centimeters and take a measurement of one of the sides on each existing fan (don't measure corner-to-corner). Your ruler will be easiest to read in centimeters but most fans are sized in millimeters until they start to hit 120mm and above. Most cooling fans are the same but there are two main jobs they've got to perform: cooling the case, or tower... and cooling the CPU, or processor. Case fans are straightforward and typically measure either 80mm or 120mm, and as I'm sure you're aware, are responsible for exhausting warm air. Let me guess -- is there a dusty green shroud that covers the processor and its cooling fan? Maybe not -- but most of the time these guys were present in 2004-era Dells (as well as today) - and the case fan would sit at the very back and pull air through. (Is the case fan attached using rubber cone fasteners? Yeah if thats the deal then those are toast, unless you're a surgeon for your day job). The rubber plugs are for noise isolation (and cost cutting) but not to worry -- your new case fan will be the strong silent type. Dell probably has a smaller Foxconn or JMC fan sitting on top of a heat sink, which is contacting the CPU (Pentium 4?). That size will probably be 50mm but my recommendation would be to determine what socket type you're using -- and with this information, you can just purchase a new modern CPU cooler (heat sink and integrated fan, maybe copper heat spreaders, yada yada). ---If you're comfortable enough to switch out fans then you can handle a Pentium cooler switcheroo.--- (An instruction manual would be supplied, btw). But you'd just pop the bracket or clips securing the heat sink to the CPU, pull it up and outta there, wipe off the exposed CPU surface with a dry microfiber -- or use wadding polish like Eagle Nevr-Dull and THEN use the microfiber, apply PEA-SIZED drop of thermal paste to surface of processor (paste comes with the cooler), clip on said cooler, and open beer. Here are some links to go look at... (there are literally trillions of online merchants selling compustuff) -- newegg.com is always a good one of course due to great cust svc and practically next-day "ground" shipping; for in-person purchasing, you can't beat Micro Center (microcenter.com), but they may not have a store nearby. Check these guys out - I've used them for Dell-specific parts before and I like how you can search for components based on socket type, processor model, etc... http://www.directron.com/cpu---coolers-cpu-coolers.html
This page will list a bunch of socket types from which to choose -- and if you're using a Pentium 4 on a 2004 or 2005 Dimension, for example, you'd probably want to click the Socket 478. If you're unsure, go to support.dell.com and plug in your service tag and pull your orig specs.
(Complete alternative: Dell Outlet Store... refurbished Core2Duo or Core2Quad with 4GB dual-channel memory, or hey I'll build you a 10-year future proof system - you pick the case design - for less $$ and will make Dell's cookie cutter systems look like.. well, cookie cutter systems.) Bestoluck!