External hard drive

Neither interface is fast enough to give you a satisfying experience.
You would be much better off getting an inexpensive eSATA Add On Card for your laptop.

As far as the hard drive goes, most likely the external hard drive speed matches the system drive. The most common hard drive speed is 7200RPMs. However, since you will be using it through USB 2, there will be a bottleneck because of the speed at which data can be transferred is limited to about 480Mbps (theoretical). Assuming you have SATA 2 drive as your system drive, transfer speeds is limited to 3Gbps.

SATA I - 1.5Gbps (1536 Mbps)
SATA II - 3Gbps (3072 Mbps)
SATA II - 6Gbps (6144 Mbps)

USB I - 1.5Mbps
USB II - 480Mbps
USB II - 4.8Gbps (4915.2 Mbps)

external USB and firewire will be SLOW in comparison to a internal HDD. Not bad if you need to read or write small (say 20MB) files but the more intense you use it the worse it gets.

I believe the Inspiron 1520 was fitted with one drive bay (check your manual!!!)
If against my belief it has a second internal unused HDD bay then buy a HDD holder at dell (or possibly somewhere else). That is a small pin extender for a sata drive to the MB of the dell. And just install a second HDD.

Second option is to replace the HDD. Easy as can be with your dell systen. Dell has online guides for this and you just need to unscrew a small "door" at the bottom of your laptop giving accessto just the HDD compartment.

The high performance (more expensive) harrddisks Dell built into the in the 1520 line are from the Seagate Momentus G-Force Protection series. The newest 7200 rpm 500 GB HDD from that series uses less power and produces less heat than the smaller capacity ones used by dell. So you can upgrade to that without any problem.
Another option would be the Western Digital WD10TPVT a 1TB (1000GB) 5200rpm drive.

You could reinstall the OS or put the original OS and everything else on the new HDD with a imaging tool.
Buy yourself a 2.5" USB enclosure to make life easier doing this and use the old HDD as a external USB HDD afterwards.

It is easy to do yourself or you can ask a local PC shop to do it for you.

If you need more than 1 TB there is no fast solution unless you have a laptop with 2 HDD bays (like the dell 1720 series has).

The given SATA and USB speeds are maximum speeds. Those are never reached. I have seen a over 100MB/sec capable HDD crawl back to 20 mb/sec or less over USB in a external enclosure. Data has to e transferred over several interfaces.

Dell and other laptops indeed can accomodate addon cards like SATA cards. My experience is however that the DELL interface of the addon card limits the speed of everything that is attached to it. So the laptop even limits the reachable speed of the ESATA card. So a harddisk over a ESATA addon card will still be slower than a internal HDD, but perhaps faster than a USB or FIREWIRE option, I never tested this.

a new hdd sounds the best option then  
thanks!

what do you think about:
‏Momentus 500GB 16MB 7200RPM Sata II‎
will it fit to my dell?

The seagate momentus series consists of many variations. Seagate Momentus G-Force Protection 7200rpm disk might be nice when you want the fastest and are afraid of dropping your harddisk.

As long as the exterior size is standard every disk should fit. Basically harddisks continually get faster (even at the same rpm's) and newer models use less power.

You can use an internal HD in one of these w/ an e-SATA connection

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

It is a very nice adapter.
In the comments one customer already wrote:
"Cons: eSATA speed is half the advertised 3 Gbps speed because of the Initio chipset used. Please see my video review for pictures of the item, board and chip. "
I almost bought one myself a week ago for a PC because I though it would be just a esata passthrough without speed reduction.

To use this one in a dell laptop a esata adapter card is also needed. That also introduces a speed drop because a conversion of interfaces. Therefore this will not be as fast a a internal HDD.

The price of both adapters combined is about as expensive as putting a new big internal HDD in a laptop.